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MicropollutantsRemovalandOperatingStrategiesinUltrafiltrationMembraneSystemsforMunicipalWastewaterTreatment:PreliminaryResults

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Micropollutants Removal and Operating Strategies in Ultrafiltration MembraneSystems for Municipal Wastewater Treatment: Preliminary Results

Paolo Battistoni,*,† Emanuela Cola,† Francesco Fatone,‡ David Bolzonella,‡ and Anna Laura Eusebi†

Institute of Hydraulics and Transportation Infrastructures, Marche Polytechnic UniVersity, Via Brecce Bianche,60131 Ancona, Italy, and Department of Science and Technology, UniVersity of Verona, Strada Le Grazie 15,Ca Vignal 37134 Verona, Italy

Membrane systems are reported to enhance the removal of micropollutants (heavy metals and organic persistentcompounds) from wastewaters. However, with regard to real municipal wastewater, where the micropollutantsare present at very low concentrations, the debate on the real convenience of operating membrane systems isstill ongoing. This paper presents the preliminary results from a pilot study where the removal of severalmicropollutants (80 compounds, grouped in the families of metals and metalloids, polynuclear aromatichydrocarbons (PAH), volatile organic compounds (VOC), halogenated volatile organic compounds (HVOC))from real municipal wastewater was studied using an ultrafiltration membrane system. With the purpose tooptimize the removal performances, the prime objective was to determine the best plant configuration, tertiaryfiltration, or membrane bioreactor, as well as the best operating parameters, with particular concern to theactivated sludge concentration. To expand the practical interest of the results for application in real plants,the sludge filterability also was studied according to different activated sludge concentrations and permeatefluxes. The objective was to estimate operating parameters able to enhance the removal of micropollutantsand optimize the ultrafiltration process. After one year of experimentation, the results that were obtainedgave important indications about the real role of the membrane system. The membrane not only demonstratedthat it could be a simple barrier against the particulate pollutants, but it also demonstrated that it can enhancethe removal of dissolved micropollutants, thanks to the “layer effect”.

Introduction

The necessity to produce treated wastewaters with high-quality standards for discharge or reuse implies the adoption ofvery effective processes in the field of wastewater treatment.Among the best-available techniques (BAT), the membranebioreactor (MBR) is supposed to be also the best choice forenhancing the biochemical process performances.1 Recently,many researchers have studied different peculiarities of MBRsand their applications,2 with reference to different topics suchas the filterability of sludge,3 the overall feasibility of thewidespread application of MBRs,4 or parameters linked to thecritical flux.5 Numerous studies have showed that the MBRshave the capacity to efficiently remove conventional pollutants(carbon, nutrients, suspended solids, pathogens), as well as heavymetals6 and organic persistent compounds, to obtain reusablewater and high quality standards.7-9 However, in regard totreating real municipal wastewater, the actual benefits ofadopting membrane technology are still controversial.6

Besides the performance, the process configuration alsoshould be considered (i.e., the choice between tertiary filtration(TF) or MBR processes), because these two schemes involvealmost different capital and operation and maintenance (O&M)costs.10,11

In light of the scenario just outlined, this paper examinesthe treatment of real municipal wastewaters by ultrafiltration(UF) membranes, operating both as TF and as a filter submergedin the activated sludge, and it focuses on numerous micropol-

lutants. Furthermore, the filterability of the activated sludge wasalso investigated. This 2-fold approach could allow one toevaluate whether an operating strategy that optimizes theremoval performances can be also industrially sustainable forthe membrane system. Therefore, the final intention of thisstudy was to draw useful considerations about the design andoperation of the membrane plants for municipal wastewatertreatment.

Materials and Methods

The Municipal Wastewater Treatment Plant (WWTP)and the Membrane Pilot Plant. The study was conductedusing a large pilot plant hosted in a full-scale municipalwastewater treatment plant (WWTP) that had a treatmentcapacity of 85 000 population equivalent (PE). Its operation unitswere fairly conventional, as shown in Figure 1 and detailed inTable 1. The membrane pilot plant adopted a ZeeWeed (GE-Zenon) hollow fiber module; the main characteristics of thispilot plant are reported in Table 2, together with the mainfeatures of the filtration chamber. As shown in Figure 2, thepilot plant was equipped with several on-line probes and metersthat measured properties such as the following: transmembranepressure (TMP), permeated/backwashed flow rates, solublechemical oxygen demand (COD), conductivity, turbidity in thepermeate, temperature, and suspended solids in the filtrationchamber.

The pilot plant could operate at fixed permeate fluxes (J),set by a program line controller (Figure 2) and a frequencyregulator for the process pump. The filtration cycle wascomposed of suction (300 s) and backwashing (30 s). At theend of each test, the original conditions of the membrane werere-established by submerging the module in hypochlorite

* To whom correspondence should be addressed. Tel.:+39 0712204530. Fax:+39 071 2204528. E-mail: p.battistoni@univpm.it.

† Institute of Hydraulics and Transportation Infrastructures, MarchePolytechnic University.

‡ Department of Science and Technology, University of Verona.

6716 Ind. Eng. Chem. Res.2007,46, 6716-6723

10.1021/ie070017r CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 08/04/2007

solution (200-600 mg Cl/L) for 4-12 h, always keeping thescouring aeration switched to “on” mode.

The membrane pilot plant was located between the secondaryclarifier and the disinfection contact tank (see Figure 1). Thischoice allowed one to (a) easily feed the membrane system withfresh effluents from the secondary clarifier, and (b) use NaClOfrom the disinfection section for the chemical cleanings of theUF membrane.

Membrane Pilot System: Tertiary Filtration (TF) and theMembrane Bioreactor-Like (MBR-like) Configuration. Theexperimental tests were performed by feeding the filtrationchamber with both secondary clarified effluent (to obtain theTF modality) and activated sludge that was thickened differentlyvia the addition of secondary effluent, according to theexperimental test (to obtain a form of MBR-like modality). Inregard to the MBR-like configuration, the objective of theexperimentation was to determine the pure effect of themembrane system, under different concentrations of mixedliqour suspended solids (MLSS), on the removal of micropol-lutants. Hence, at the beginning of each test, the filtrationchamber was fed with activated sludge that was taken from thefull-scale plant and the test lasted for 4 days. According to thisapproach, the characteristics of the activated sludge in the pilotplant and the full-scale WWTP were always similar.

Sampling and Analysis Methodology.Two types of auto-matic samplers were used to collect composite samples ofwastewaters over a period of 24 h: the first was conventionaland the second was expressly designed for the experimentation(it was equipped with a ZeeWeed10 membrane module). Thetwo samplers allowed one to obtain samples, averaged over 24h, for the determination of the total concentration of pollutantsand their distribution in the liquid and solid phases. The sampleswere collected from the main streams of both the full-scaleWWTP and the pilot plant. In particular, daily averaged sampleswere taken from the influent sewage, the effluent from theprimary treatment, the waste-activated sludge (WAS), theeffluent from the full-scale secondary clarifier (which was theinfluent to the pilot plant), and the membrane permeate. Thefirst screening for the characterization of the wastewaterconsidered the determination of some 80 parameters, accordingto the United States Environmental Protection Agency (USEPA)methods.12 Among the compounds, heavy metals, volatileorganic compounds (VOCs), halogen volatile organic com-pounds (HVOCs), and polynuclear aromatic hydrocarbons(PAHs) were detected.

Parallel and Contemporary Tests.Two types of paralleland contemporary tests were performed. The pilot plant thatwas operating as a MBR-like reactor at 5 g MLSS/L was usedin parallel to (i) the previously mentioned UF sampling systemin TF modality, in the Type 1 test; and (ii) a laboratory-scalefiltering system that was equipped with a cellulose mem-brane filter (with nominal pore size of 0.45µm), in the Type 2test.

Results and Discussions

Micropollutants in the Main Stream of the Full-ScaleWWTP. To compare the performances of the membrane systemand the conventional WWTP, the micropollutant concentrationswere determined in the main streams of both processes. Greatvariability of the influent concentrations of hazardous com-pounds was observed, as a consequence of nonidentifiablereasons. Therefore, Table 3 reports the ranges of concentrationgrouped by the family of compounds that may well show thebasic roles of the different operation units. Generally, theoccurrence and removal of metals was consistent with the datareported for other municipal WWTPs that treat municipal ormunicipal/industrial wastewaters.13-15 In regard to the PAHs,the influent wastewater showed contents that were lower thanthose of other cases;16,17 nevertheless, the concentrations wereon the order of micrograms per liter (µg/L) and allowed us tosuppose the presence of industrial discharges in the catchmentsarea.18 The volatile compounds were almost organic solventsthat had been probably discharged from local factories into thesewer system; their occurrence were very variable but always<20 µg/L.

Organic Persistent and Hazardous Compounds.Theinfluent raw wastewater originated primarily from a urban area.The compounds present in the greatest amounts were asfollows: benzene, toluene, and xylenes (BTEX) (1.31-2.15µg/L) among the VOCs; naphthalene (0.091-0.126µg/L), fluorene(0.022-0.074 µg/L) and phenanthrene (0.033-0.047 µg/L)among the PAHs; and chloroethylenes and methylene chlorides(0.9-8.95 µg/L) among the HVOCs. With specific referenceto this class of compounds, the first significant removal ofVOCs, PAHs, and HVOCs occurred already in the physicalheadworks (screening, aerated degritting, and primary sedimen-tation). The effect of these processes can be more or lessimportant, depending on the characteristics of those compounds,

Figure 1. Full-scale wastewater treatment plant (WWTP) and location ofthe membrane pilot plant.

Table 1. Main Features of a Full-Scale Wastewater Treatment Plant(WWTP)

parameter value

primary circular settlernumber 2volume 1325 m3

biological processnumber of lines 2total volume, pre-denitrification 3500 m3

total volume, nitrification 2450 m3

air blowers 4total power 240 kWsecondary settler with radial flow

number 2volume 1608 m3

Table 2. Main Characteristics of the Membrane Pilot Plant

characteristic value

membrane model ZeeWeed 500module dimensions 1000 mm× 700 mm× 200 mmnominal pore size 0.04µmfiltration type out to inmodule configuration submergedmembrane type hollow fibermembrane area 21.6 m2

filtration chamber volume 1400 Lanalysis tank volume 210 Lpermeate tank volume 145 L

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076717

as well as the solid/liquid partition coefficient and their volatility(see Table 4). In particular, volatile compounds (VOCs andHVOCs) were almost completely dissolved and were easilystripped by blowing air and water movements. On the otherhand, PAHs are both dissolved in the liquid phase and associatedwith the suspended particulate, according to the octanol-waterpartition coefficient (kow). This behavior is consistent with thefact that PAHs are rather hydrophobic and, therefore, have atendency to associate with the particulate matter. Therefore,PAHs can be removed in the physical/mechanical headworksboth by gravity separation, if linked to the suspended particulate,and by volatilization, if very volatile (i.e., naphthalene).17,19Onthe other hand, VOCs can be removed in the biological processtank, as a result of the volatilization in the aerated tank. Incontrast, no remarkable effects were observed for the PAHs,probably because these compounds are present at very lowconcentrations already in the effluent of primary sedimentation.Unexpectedly, HVOCs remained at the same concentration,while an additional removal by stripping and biodegradationwas expected. Therefore, it could be reasonably assumed that,in the municipal WWTPs, the possible volatilization of HVOCscould be almost completed before the secondary biologicaltreatments.

The effect of the UF membrane was significant for theremoval of the organic micropollutants, because of its barriereffect. HVOCs underwent the most important removal. Thisevidence, when coupled to the low removal observed in thefull-scale conventional biological process, indicates that HVOCsare partially recalcitrant to biodegradation but can be retainedby the sieving effect of the UF membrane. Unfortunately, theHVOC contents in the activated sludge (on dry basis) werealways below the limits of quantification and did not allow usto verify the mass balances.

Metals. Heavy metals and metalloids (Cu, Cd, Hg, Cr, Ni,Pb, As) observed in the influent raw wastewater are mainlyrepresented by Cu (13-94 µg/L), Cr (7-45 µg/L), Ni (4-51µg/L), and Pb (5-15 µg/L), whereas As, Hg and Cd usuallyshowed a minor occurrence (typically<1 µg/L). Also, in thiscase, the preliminary treatments could have an important role,both in degritting and in primary sedimentation. This fact waspartially in agreement with the partitioning between the liquidand solid phases: the maximum soluble fraction (∼90%) wasobserved for Ni and the minimum (<4%) for Hg. Differentlyfrom the fate of organic micropollutants, the activated-sludgeprocess had a significant effect on the removal of metals, mainlybecause of the sorption/precipitation on the biomass and/or theextracellular polymeric substances (EPS). This behavior hasbeen widely observed and reported in the literature.20,21 Theresulting concentrations of heavy metals in the activated sludge,expressed in terms of mg (kg SS)-1, are shown in Table 5.Because of their concentration in the sludge, a low presence ofsuspended solids in the effluent also can result in a significantconcentration of heavy metals in the effluent. With specificreference to the presence of heavy metals in the effluent, it

Figure 2. Pilot-plant flow scheme.

Table 3. Micropollutants Ranges in the Main Streams of the Full-Scale WWTPa

Range of Micropollutant (µg/L)

samples taken from:influent rawwastewater

outflow from thephysical headworks

effluent fromfull-scaleWWTP UF permeate

metalsb 18.00-265.00 9.00-112.00 7.70-93.60 4.70-56.00VOC 0.65-7.10 0.23-2.98 0.12-0.80 0.01-0.25PAH 0.23-0.67 0.08-0.27 0.07-0.26 0.05-0.16HVOC 1.45-12.17 0.92-5.59 0.23-5.08 0.05-0.52

a Data refer to the total concentrations over the entire experimental period; membranes are applied both as tertiary filtration (TF) and as MBR-like.b Includes Cu, Cd, Hg, Cr, Ni, Pb, and As.

Table 4. Organic Micropollutants Partition in the Liquid Phase

micropollutant

percentage ofcompounds in

the liquid phase

volatile organic compounds, VOCs >99polyaromatic hydrocarbons, PAHs

log kow < 5.0 <50log kow > 5.0 <6.0

halogen volatile organic compounds, HVOCs >99

6718 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

should be noted that the most persistent are Crtot, Ni, and Cu,while Pb is present in a minor amount.

In regard to the overall effect of the membrane system, afurther removal of∼40%-50% was observed. This can beattributed to the sieving effect, as well as other phenomena thatare better discussed in the following section of the paper.

Optimization of Heavy-Metals Retention: MBR or TF?After the macroscopic effect of the UF membrane on theremoval of priority pollutants was estimated, the role of theUF membrane was better investigated. The effect of the MBR-like and TF configurations is described and discussed in thissection. For this purpose, metals that were used in operation ofthe plant were monitored according to the configuration shownlater in this paper in Figure 5. However, note that, during TFtests, the concentration of activated sludge into the membranetank led to values of 1.5 g/L, whereas in the MBR-like modality,the concentrations were in the range of 2.5-10 g/L and werekept stable at the desired levels by adjusting the influent flowrates. Furthermore, it is not worth noting again that, accordingto the methodology of the test previously described, the MBR-like configuration reproduced only the impact of differentactivated-sludge concentrations on the membrane role. In fact,the characteristics of the activated sludge, both physicochemicaland microbiological, were typical of the conventional activated-sludge process from which the sludge originated.

Twelve experimental tests were performed, always treatingthe real liquor effluent from the secondary clarifiers of theWWTP. Table 6 shows the concentrations of heavy metalsinfluent to the membrane filtration system and the averageremovals obtained in MBR and TF configurations, respectively.The TF configuration showed satisfactory metals removal(17%-70%), but the value always was less than that observedin the MBR-like configuration. This particular observationsuggests that, besides the barrier effect exerted by the membrane,the fouling and/or cake layer on the membrane area may havea role on the removal of metals in the liquid phase. Furthermore,because the hydrodynamic conditions were the same, the higherremoval performances (increases in the range of 8%-47%) ofthe MBR configuration indicated that different biomass con-centrations enhance the layer effect. This fact suggested thatan equilibrium occurred between the metal ion concentrationin the aqueous phase and the metal in the sludge, probably onor close to the membrane surface, where a higher concentration

of heavy metals is observed as an effect of concentration-polarization phenomena and fouling and/or cake layer formation.

To confirm the previous results and to better investigate thesieving and the layer effects, contemporary and parallelinvestigations were performed, according to the method de-scribed in the previous section.

The comparison between the different configurations of themembrane systems is reported as proportional values and,therefore, can be simply calculated, according to eq 1:

whereCe is the concentration of metal in the effluent from theTF andCp is the concentration of metal in the effluent fromthe MBR-like configuration.

Results reported in Table 7 confirm that the MBR modalityis more effective than the TF filtration. Furthermore, althoughconsiderations are possible only for Cu and Ni, which werealways over the detection limits, the benefits of the MBR-likemodality, with respect to the TF configuration, are comparablein test types 1 and 2. This experimental evidence may indicatethe major effect of the layer on the role of the membrane system,which seems to be dependent on neither the membrane materialnor the pore size.

Consideration on the TMP Decline: TF and MBR. Togeneralize the results also for a possible industrial applicationof the system, the results about micropollutants removal mustbe coupled with considerations about the fouling of themembranes in TF and MBR configuration. Therefore, a detailedcampaign of filtration tests was conducted to study themembrane fouling rates, which are defined as the decline ofTMPs over time (or increase as TMP’s absolute value). Thefinal objective of this activity was to study the conditions thatboth optimize the membrane life and operation and, at the sametime, enhance the removal of the micropollutants.

To evaluate the long-term performances, each filtration testlasted four days or more (except for test 1). This approachallowed the steady-state fouling rates to be taken into consid-eration, avoiding the need to consider the initial conditioningfouling11 in the calculations. At the end of each test, the initialconditions of the membrane surface were re-established bychemical cleaning, according to the protocol described in the“Materials and Methods” section of this paper.

The TMPs of the TF operation are reported in Figure 3,whereas Table 8 reports the steady-state fouling rates. Consider-ing that the critical flux for this type of membrane was reportedto be in the range of 17-30 LMH5 (where the term “LMH”means liters per square meter per hour), tests 1, 2 and 3 clearlywere conducted under stressed conditions, whereas tests 4 and5 were performed under conditions of subcritical flux, wherelow or null fouling rates are generally observed. Here, theconditioning fouling lasted the first day, when the TMP declinewas less severe, whereas the steady-state fouling rate wasobserved from the second day onward. As expected, the higherthe permeate flux, the faster the membrane fouling; however,unexpectedly, the net flux (34 LMH) involved a lower TMPdecline (0.028 bar/d).

Operating the direct filtration of the activated sludge (MBR-like), the membrane permeability was studied by applyingdifferent permeate fluxes, in the range of 7-20 LMH, accordingto different suspended solids concentrations (in the range of0.5-10 g MLSS/L. According to the scale of the experimenta-tion, also in this case, the objective was to study the macroscopic

Table 5. Heavy Metals in the Activated Sludge

elementaverage

(mg/kg SS)standard deviation

(mg/kg SS)

As 3.6 1.3Hg 2.1 1.7Cu 347.8 28.6Pb 61.1 11.2Cd 1.2 0.6Ni 106.9 48.3Cr 525.1 129.1

Table 6. Removal of Metals from the Dissolved Phase: MBR-likeand TF Modality

Percentage Removal (%)

metalinfluent concentration

range (µg/L) MBR-like TF

Cu 5.6-75.0 81 50Pb 3.5-11.0 74 34Cd 0.4-7.7 64 17Ni 3.4-90.0 41 33Cr 2.8-40.0 85 73

TSS range 2500-10000 mg/L 5-1500 mg/L

modality)Ce - Cp

Ce× 100 (1)

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076719

phenomena. Figure 4 shows the TMP trends during the test at0.5, 2, 5, and 10 g MLSS/L, and Table 9 reports the main resultsobtained. As a general and most-evident remark, the higher thepermeate flux, the more evident the impact of the MLSSconcentration on the fouling phenomena.

Although no relevant changes were observed when thepermeate flux was much lower than the critical value (7 LMH(see Figure 4a)) and approaching the critical flux (e.g., 14 LMH(see Figure 4b)), the membrane was more sensitive to the MLSSconcentration. As a result, passing from 2 g MLSS/L to 5 gMLSS/L, the TMP decline increased from-0.006 bar/d to-0.03 bar/d.

However, no further decrease of the TMP decline wasobtained when passing from 5 g MLSS/L to 10 g MLSS/L (seecurves C and D in Figure 4b). Therefore, among the MLSScontents typically applied for municipal wastewaters treatment,certain values in the range of 2-5 g MLSS/L were supposedto mark the borderline between weak and severe foulingphenomena. Furthermore, when the permeate flux was supposedto be close to the critical flux (e.g., 20 LMH (see Figure 4c)),the TMP decline was not remarkably influenced by the MLSSconcentration. However, its effect was clearly visible in thestarting TMP (that is, the initial pressure) soon after thetriggering of the filtration.

All experimental results can be simply summarized byplotting the membrane permeability (Js) versus the net flux (J),

whereJs is calculated considering the average TMP over days2-4:

Therefore, the pure effect of the MLSS concentration on thesludge filterability is visible from Figure 5, where the curves at2, 5, and 10 g MLSS/L are plotted.

Figure 5 confirms that the filterability of the sludge is affectedby the MLSS content if the concentrations pass from 2 g/L to5 g/L, whereas a MLSS concentration from 5 g/L to 10 g/L hassimilar impacts on the UF process. In light of the results justdescribed, the correct compromise between the activated-sludgeconcentration and the permeate flux must be pursued, to achievethe industrial and economical sustainability of the process. Asa result, from one side, biomass concentrations up to 2 gMLSS/L can increase the membrane permeability, so thisstrategy can reduce the capital and O&M costs; from the otherside, higher concentrations might enhance the removal ofmicropollutants. However, in this last case, overcritical condi-tions are observed and more-frequent chemical cleanings of themembranes are required, which deteriorates the membrane life.

As a general preliminary consideration, the application ofhigher fluxes seems to be convenient, but further investigationsare needed to better evaluate the membrane life and real impacton micropollutants removal.

Conclusions

The main remarks of this study can be summerized as follows:(1) The physical/mechanical headworks (screening, sieving,

degritting, and primary sedimentation) have an important rolein the volatilization and/or gravity separation of numeroushazardous compounds present in raw municipal wastewater.

Figure 3. Behavior of the membranes working in tertiary filtration (TF) modality. Table 5 reports the fouling rates.

Table 7. Metals Removal: Comparison between MBR and TT Modality

Test Type 1 Test Type 2

metalconcentrationrange (µg/L) [(Ce - Cp)/Ce] × 100 (%)

concentrationrange (µg/L) [(Ce - Cp)/Ce] × 100 (%)

As <2 nda <2 nda

Hg <2 nda <2 nda

Cu 16-52 69 5.6-10.6 60Pb 5-7 20 <2.5 nda

Cd <0.25 nda <0.25 nda

Ni 20-29 18 3.4-9.5 24Cr 9-11 32 <2.5 nda

a Not detected.

Table 8. Filtration Tests in TT Configuration

testpermeate flux

(LMH)TMP decline

(bar/d)MLSS concentration

(g/L)

5 7 -0.001 0.0144 14 -0.001 0.0173 20 -0.055 0.0132 27 -0.063 0.0251 34 -0.028 0.055

Js(permeability @ 20°C) )J (net flux)

TMPaveragesdays 2-4/ (2)

6720 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

(2) The biological process of a conventional activated sludgewastewater treatment plant (WWTP) has a major role in theremoval of the heavy metals, even operating at SRT of 10-15days, which are time periods that are commonly applied inconventional plants. Also, an effect on the volatile organiccompounds (VOCs) was observed in the full-scale bioreactor.In contrast, the polyaromatic hydrocarbons (PAHs) were alreadypresent at very low concentrations in the effluent of the primarysedimentation and the biological process did not perform furthermacroscopic removals. Unexpectedly, the halogen volatileorganic compounds (HVOCs) did not undergo significantremoval, probably because of their recalcitrance to biodegrada-

Figure 4. Membrane bioreactor (MBR) modality, shown using transmembrane pressure (TMP) profiles versus time at different fluxes ((a) 7, (b) 14, and(c) 20 LMH) and concentrations of mixed liquor suspended solids (MLSS).

Figure 5. Plots of membrane permeability (Js) versus permeate flux (J) atdifferent MLSS contents.

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076721

tion within a time period of 10-15 d, and they are present inthe form of small molecules that are not prone to biosorption.

(3) The sieving effect of the UF membrane system led to avirtually complete removal of the HVOCs. As for the heavymetals, the fouling and/or cake layer had a major role and ledto further removal in the range of 40%-50%. This effect isdifferent depending on whether the membrane system isfiltrating the secondary effluent or the activated sludge directly.In particular, the presence of activated sludge with MLSS in aconcentration of 5-10 g/L can enhance the layer effect andincrease the removal performances proportionally (18%-69%).Currently, further reasonable considerations about the removalmechanisms are not possible, because the concentration ofmetals in the liquors are close to the limits of quantification;

(4) Fouling phenomena are more sensitive to MLSS concen-trations when the flux is similar to or higher than the criticalvalue. In particular, either the transmembrane pressure (TMP)declines or the starting values are influenced. Among the typicalMLSS concentrations applied for the treatment of municipalwastewaters (3-10 g/L), values in the range of 2-5 g MLSS/Lmark the borderline between weak and severe fouling phenom-ena;

(5) Higher MLSS concentrations seem to enhance the removalof micropollutants, but also worsen the fouling phenomena.Moreover, the application of higher permeate fluxes seems tobe convenient, but, as a consequence, the membrane life canbe severely reduced. As a matter of fact, currently, using MLSSin a concentration of∼5-10 g/L and a permeate flux of 15-20 LMH seems to be a good choice to enhance the layer effectand have a sustainable operation of the membrane system.However, also in this case, further investigations are still needed.

Nomenclature

AbbreViations

BAT ) best-available techniquesBOD ) biological oxygen demandCOD ) chemical oxygen demandEPS) extracellular polymeric substancesHVOC ) halogen volatile organic compoundsLMH ) liter per square meter per hour (measure of flux)MBR ) membrane biological reactorMLSS ) mixed liquor suspended solidsO&M ) operation and maintenanceP&Id ) piping and instrumentation diagramPAH ) polynuclear aromatic hydrocarbonsPE ) population equivalentTF ) tertiary filtrationUF ) ultrafiltration

VOC ) volatile organic compoundsWAS ) waste-activated sludgeWWTP ) wastewater treatment plant

Variables

J ) permeate fluxJs ) membrane permeabilitykow ) octanol-water partition coefficientSRT ) sludge retention timeTMP ) transmembrane pressureTSSout ) total suspended solids in the retentate flow rate

Acknowledgment

The authors thank the Italian Ministry of the University andResearch for the financial support of this research through theprojects “PRIN 2003” and “PRIN 2005”. Multiservizi SpA isalso kindly acknowledged to have hosted the pilot plant in theFalconara-Vallechiara WWTP.

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(12) http://www.epa.gov/epahome/index/.(13) Busetti, F.; Badoer, S.; Cuomo, M.; Rubino, B.; Traverso, P.

Occurrence and removal of potentially toxic metals and heavy metals inthe wastewater treatment plant of Fusina (Venice, Italy).Ind. Eng. Chem.Res.2005, 44, 9264-9272.

(14) Santarsiero, A.; Vecchietti, E.; Donati, G.; Ottavini, M. Heavy MetalDistribution in Wastewater from a Treatment Plant.Microchem. J.1998,59, 219-227.

(15) Chipasa, K. B. Accumulation and fate of selected heavy metals ina biological wastewater treatment system.Waste Manage.2003, 23, 135-143.

(16) Villar, P.; Callejon, M.; Alonso, E.; Jime´nez, J. C.; Guiraum, A.Temporal evolution of polycyclic aromatic hydrocarbons (PAHs) in sludgefrom wastewater treatment plants: comparison between PAHs and heavymetals.Chemosphere2006, 64, 535-541.

(17) Vogelsang, C.; Grung, M.; Jantsch, T. G.; Tollefsen, K. E.; Liltved,H. Occurrence and removal of selected organic micropollutants at mechan-ical, chemical and advanced wastewater treatment plants in Norway.WaterRes.2006, 40, 3559-3570.

Table 9. Filtration Tests in MBR Configuration

testpermeate flux

(LMH)TMP decline

(bar/d)MLSS concentration

(g/L)

I 7 -0.0047 0.4II 7 -0.0077 0.8III 7 -0.0056 1.5IV 7 -0.0076 2.2V 7 -0.0066 8.8

VI 14 -0.0074 0.8VII 14 -0.0061 1.6VIII 14 -0.0269 5.4IX 14 -0.0134 12.5

X 20 -0.0449 0.6XI 20 -0.0210 1.7XII 20 -0.0533 4.2XII 20 -0.0328 9.1

6722 Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007

(18) Palmquist, H.; Hanæus, J. Hazardous substances in separatelycollected grey- and blackwater from ordinary Swedish households.Sci. TotalEnViron. 2005, 348, 151-163.

(19) Katsoyiannis, A.; Samara, C. Persistent organic pollutants (POPs)in the sewage treatment plant of Thessaloniki, northern Greece: occurrenceand removal.Water Res.2004, 38 (11), 2685-2698.

(20) Battistoni, P.; Fava, G.; Ruello, M. L. Heavy metals shock load inactivated sludge uptake and toxic effect.Water Res.1993, 27 (5), 821-827.

(21) Brown, H. G.; Hensley, C. P.; McKinney, G. L.; Robinson, J. L.Efficiency of heavy metals removal in municipal sewage treatment plantsEnViron. Lett. 1973, 5, 103-114.

ReceiVed for reView January 5, 2007ReVised manuscript receiVed June 22, 2007

AcceptedJune 23, 2007

IE070017R

Ind. Eng. Chem. Res., Vol. 46, No. 21, 20076723