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A hybrid flotation–microfiltration cell for effluent treatment

E.N. Peleka*, P. Mavros, A.I. Zouboulis, K.A. Matis

Division of Chemical Technology and Industrial Chemistry, Department of Chemistry,Aristotle University of Thessaloniki. email: peleka@chem.auth.gr

Received 20 June 2008; accepted 26 August 2008

Abstract

Air bubbling is one of the techniques used to limit membrane fouling. Under appropriate conditions, it couldbe used also as the transport means for flotation, which is another efficient separation method applied in watertreatment. The present paper reviews briefly the integration of both processes in the same tank, taking advantageof dispersed-air flotation combined with microfiltration (MF) by submerged membranes. Process parametersinvestigated were the nature, size and concentration of solid particles (zeolite, hydrotalcite, zinc hydroxide andiron colloidal particles), the type and concentration of selected ions (Zn2+, P–PO4

3�), the effect of gas flowrate, as well as the frequency and length of backflushing, which reduces the fouling of membranes.

Keywords: Bonding agents; Flotation; Heavy metal-ions removal; Hybrid process; Membrane filtration

1. Introduction

The polluter pays principle ensures nowadays,at least in principle, that companies have to pay thecosts associated with environmental damage or atleast in part. Therefore, from their point of view,an economic and effective solution is requiredby developing techniques for improved treatment.On the other hand, many of the metal ions (cationsor oxyanions) which are target compounds and areoften present in effluents, could be further concen-trated for recycling, avoiding incineration of thetoxic sludge, since this corresponds to transferring

pollution from one state to another. If we succeedin re-using water and metal ions, ‘‘end-of-the-pipe’’ solutions will no longer be necessary.Attempts have been made, therefore, to developa more specific water treatment technique toprovide industrial process water as close as possi-ble to the source, applying quality assuranceand efficiency control techniques for operationalmanagement. An innovative and cost-effectiveapproach minimises environmental impact fromwastewater and prevents potential damage tohealth (Erwe et al., 2005).

Several industrial wastewater streams maycontain heavy metal ions, which must be satis-factorily removed, before the treated watersmay be discharged or reused. Various bondingmaterials, presenting selectivity and fast reaction

*Corresponding author.Presented at the 2nd Conference on Small and Decen-tralized Water and Wastewater Treatment Plants(SWAT), Skiathos Island, Greece, May 2–4, 2008

0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V.doi: 10.1016/j.desal.2008.08.007

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kinetics for the removal of metals, have beenexamined for this purpose and among them, zeo-lites (Matis et al., 2004a). Dispersed-air flotationwas applied for the separation of metal-loadedzeolite particles. This combined process (termedsorptive flotation) involves the preliminaryscavenging of metal ions by using the appropri-ate sorbent particles, usually present as ultrafineparticulates, followed by flotation for the effec-tive separation downstream of them (Zouboulisand Matis, 1997).

Industrial ion-exchange resins (Lewatit-type)were also tried for the removal initially of zincions and were later tested for a copper mineeffluent; almost 100% of metal was removed.Flotation was subsequently applied for thesolid/liquid separation of zinc-loaded resin.More than 95% of the used resin was recoveredin a single stage (Zouboulis et al., 2005). Theregeneration of sorbents was studied withrespect to the desorption of metal and to theelution of surfactant.

1.1. Membranes filtration

The use of membrane systems has experi-enced exponential growth over the last 10 yearsdue to their ability to deliver drinking water andwastewater to meet stringent compliance stand-ards. With their modular design and sophisticatedautomation, membrane plants are now being builtto deliver safety and flexibility with minimaloperator intervention. Another driving force forthe popularity of membranes is that their cost issteadily decreasing. Lower costs are spurred byincreased competition, increased demand thatfavour scale of economics for manufacturing,and more efficient process operation.

Membrane systems are becoming the mainfocus of water treatment and the choice of mem-brane, module configuration, process and oper-ating parameters and pre-treatment amongstothers are very important in the efficiency ofseparation. They are very promising because of

their potential to remove particles, includingmicroorganisms, organic pollutants and inor-ganic salts, and to achieve biologically stablewater to limit microbial regrowth in the distribu-tion system.

Decrease in the permeate rate, during themembrane process, is attributed to membranefouling and is recognized as the main problemin the application of membrane technologies.Several types of fouling can occur in membranesystems, including inorganic fouling, particulateand colloidal fouling, organic fouling and bio-fouling. Pore blocking and cake formation areconsidered as the two main mechanisms ofmembrane fouling, while other factors such asadsorption, particle deposition within the poresand changes to the cake layer affect membranefouling through the modification of either orboth mechanisms (Bai and Leow, 2002).

Successful operation of membrane plantrequires careful management of fouling of themembrane; its avoidance is probably not possi-ble, but its impact can be limited by a variety oftechniques. Many researchers have tried to over-come this obstacle, using chemical or physicalmethods (Wakeman and Williams, 2002; Uedaet al., 1997) (Table 1). Chemical methodsinvolve precipitation, coagulation/flocculation,use of disinfectants or anti-scalants, choice ofmembrane materials, membrane surface modifi-cation and cleaning reagents. Physical treatmentmethods include pre-filtration, the use of turbu-lence promoters, rotating/vibrating membranes,pulsed/reversed flow, electrical fields, the peri-odical hydraulic or mechanical cleaning or theuse of gas sparging. The latter consists of a gasstream injected into the membrane module; aswarm of rising bubbles flows past the mem-brane surface, and these bubbles act as mem-brane cake scrubbers. At the same time, theturbulent flow pattern induced by the bubblesprevents the particles from approaching anddepositing on the membrane surfaces. Thus,gas sparging reduces fouling by preventing or

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limiting particle deposition and concentrationpolarization. Table 2 summarises the effect ofgas sparging on the effectiveness of MF. The per-centage of process improvement depends on thekind and concentration of treated suspension,the particle size and the liquid and gas flow rates.

1.2. Flotation

Flotation selectively separates surface-activecompounds from a solution, collecting them atthe gas–liquid interface and thereby concentratingthem at the surface. The process is widely appliedfor the removal of various species from water, forexample organics, metal ions, oils, powders, resid-ual reagents and so on, as well as for the benefici-ation of mineral particles in minerals technology.

Of particular importance to the treatment of aque-ous streams are the following flotation types:

• Sorptive flotation: It involves a preliminaryscavenging stage, with metal ions beingadsorbed on suitable particles. The sorptionphase is followed by flotation which separatesthe metal-ion-loaded sorbent particles fromthe aqueous stream, resulting in a cleanedstream.

• Precipitate flotation: The undesired ions arefirst precipitated, for example by a suitablepH adjustment, and the minute particles aresubsequently floated.

• Adsorbing colloid flotation: It is a special caseof sorptive flotation, involving the removal ofthe solute from the aqueous solution byadsorption on, co-precipitation with or even

Table 1

Techniques to improve microfiltration

System Method Type ofmembranes

Improvementin systemeffectiveness (%)

References

Raw water Membrane cleaning Hollow fibre,polyethylene

77 Mo and Huang,2003

Sulfatepolystyrene latex

particles

Ultrasonic cleaning Ceramicmembranes

95 Lamminenet al., 2004

Dispersion oftitanium dioxide

Backflushing Zirconia layerdeposited on theinternal surfaceof the tubular a-alumina support

150 Miculaseket al., 2000

Humic acidsolution

Hybrid ozonation–MF system Ceramic, flat sheet 90 Schlichteret al., 2000

Zeolite suspension Hybrid flotation–MF system Ceramic, flat sheet 200 Matis et al.,2004c

Hydrotalcitesuspension

Hybrid flotation–MF system Ceramic, flat sheet 190 Peleka et al.,2005

Zinc hydroxidesuspension

Hybrid flotation–MF system Ceramic, flat sheet 2000 Peleka et al.,2006a

Ferric flocssuspension

Hybrid flotation–MF system Ceramic, flat sheet 385 Peleka et al.,2006b

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occlusion in a carrier floc, produced by addingfrom ferric or aluminum salts. A second stageinvolves the combination of flocs into largeraggregates, which are then floated with thehelp of an appropriate surfactant (Matis,1995; Matis and Mavros, 1991).

The main disadvantage of flotation is thatsometimes its removal efficiency is limited,due to the fact that some substances either arenot retained in the froth or are not sufficientlyhydrophobic, thus remaining in the bulk disper-sion/solution (Bl€ocher et al., 2003).

Both flotation and membrane filtration areseparation processes studied extensively inliterature, but their combination in a hybrid flo-tation-membrane filtration system is an innova-tive process. The hybrid flotation-membranefiltration system combines the advantages ofboth these processes, while overcoming someof their limitations. During its operation, a sus-pension of fine-sized adsorbent is fed into thehybrid cell, where the solid particles are partiallyremoved by flotation, while clean water is

obtained from the membrane module. Some ofthe solid particles remaining in the dispersionare deposited on the surface and the pores ofthe membrane, forming a cake that graduallyblocks the pores and causes membrane fouling.The rising gas bubbles are used for the flotationprocess, while at the same time acting as mem-brane-surface scrubbers, effectively removingthe deposited cake and countering the foulingproblem.

The hybrid cell hydrodynamics was alsoexamined (Matis et al., 2004b). One of theimportant parameters in chemical units designis the flow pattern inside them; the liquid charac-teristics, the size and the scale of unit, may bethe baffles and the impeller are among the fac-tors that influence the flow pattern. As itappeared, the tanks-in-series model for N = 1without air bubbling was not satisfying andanother model should be applied. Nevertheless,the hybrid flotation-membrane system alwaysuses air; and in presence of air, a well-mixedvessel model can correctly describe it.

Table 2

Effect of gas sparging on microfiltration process

Suspensionconcentration(g L�1)

Particlesize (m)

Type ofmembranes

UL (m s�1) UG (m s�1) Improvementin systemeffectiveness (%)

References

Yeast, 0.01–10 4.2 Tubular,PVDF

0.36–1.8 0.18–1.02 135 Sur andCui, 2001

Biomass, 5 1 Hollow fibre,polypropylene

0.1–0.4 0–1.6 100 Chang andFane, 2000

Yeast, 5–20 5 Ceramic, flatsheet

0.3–1.4 0–0.8 280 Mercier-Boninet al., 2000

Zeolite, 0.5–5 3–6 Ceramic, flatsheet

6.7� 10�5 8.5� 10�4 29 Matis et al.,2004

Hydrotalcite, 2 10–20 Ceramic, flatsheet

1.68� 10�4 0–6.47� 10�3 103 Peleka et al.,2005

Ferric flocs, 0.2 208 Ceramic, flatsheet

1.68� 10�4 0–6.47� 10�3 1500 Peleka et al.,2006b

Zinc hydroxide,0.05

90 Ceramic, flatsheet

1.68� 10�4 0–6.47� 10�3 125 Peleka et al.,2006a

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This innovative cell was then tested withaqueous solutions simulating an effluent, aswell as a real effluent from an open pit coppermine (Assarel-Medet, Panagyurishte and Bulga-ria) (Lazaridis et al., 2004). A combined processof electroflotation, dissolved-air flotation/bio-flotation and membrane separation was also suc-cessfully tested, in realistic industrial conditions,using existing flotation units (Tvaruzek et al.,2003); hydrophobic polypropylene and hydro-philic polyethylene hollow fibre MF membraneswere used in that case.

Pilot plant work was performed in situ at thecopper mine (Nenov et al., 2008). Flotation recov-ered ca. 90% of the solids, with a copper content inthe froth concentrate approaching 6%. The resid-ual heavy metal (Cu, Mn, Fe and Pb) concentra-tions in the membranes permeate was below0.05 mg L�1.

Experimental details

The hybrid cell consisted of a cylindrical flo-tation column made of Plexiglas (i.d.DC = 100 mm) equipped with a flat ceramicporous gas sparger with an average porosity,Dpor, of 10–16mm, placed at a distance of20 mm from the flat bottom of the column(Peleka et al., 2006a). A membrane module, con-sisting of a twin set of parallel, double-sidedceramic membranes with a flat sheet multi-chan-nel geometry, was positioned 60 mm above thegas sparger. The membranes were hydrophilic,and had a mean pore size of 0.3mm and a totalsurface area of 0.021 m2 (see also Fig. 1).

A suspension of fine-sized particulate matterwas prepared in a mixing tank, and was then fedat a constant flow rate into the hybrid cell using aperistaltic pump, while air was introduced intothe cell through the sparger. Clean water perme-ate was drawn at a constant flow rate from themembrane module using another peristalticpump. The flow rate of the clean water and thepressure drop through the membrane (termed

transmembrane pressure (TMP)) were continu-ously monitored.

In the hybrid sorptive flotation–MF experi-ments:

1. a small amount (5 g L�1) of synthetic type-Azeolite (Ineos Silicas Ltd, Warrington, Chesh-ire, UK), with a chemical formula Na2O.2SiO2.Al2O3.nH2O, was dispersed in a mixing tankcontaining a zinc solution with an initial Zn2+

concentration of 50 mg L�1; the pH of the re-sulting dispersion varied from 10.0 to 10.5;

2. a small amount (2 g L�1) of synthetic hydro-talcite (Ineos Silicas Ltd, Warrington, Chesh-ire, UK), with a chemical formulaMg6Al2(OH)16 CO3.4H2O was dispersed in amixing tank containing a phosphate solutionwith an initial concentration of 10 mg L�1.

In the case of the hybrid zinc precipitateflotation–MF experiments, zinc was precipitated

Fig. 1. Photograph of the hybrid flotation–microfiltration cell.

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from an initial solution containing a concentra-tion of 50 mg L�1 of Zn2+ as hydroxide by adjust-ing the solution pH to&9. For the flotation phaseof the process, hexadecyl-trimethylammoniumbromide (HDTMA–Br) was used as collector,to render the zinc precipitate or the zinc-loadedzeolite particles hydrophobic, at a concentrationof 10 mg L�1. In the hybrid experiments withhydrotalcite the used collector was sodium oleateat a concentration of 200 mg L�1. In the case ofthe hybrid adsorbing colloid flotation–MF, theinitial solution contained 200 mg L�1 of Fe(III)and 10 mg L�1 of PO4

3� at pH 5. The iron flocsformed were rendered hydrophobic with theaddition of sodium oleate at a concentration of500 mg L�1.

3. Discussion of results

The effectiveness of the hybrid process wasexamined with two goals in mind (Matis et al.,2004):

• to remove from the aqueous effluent anyundesirable dissolved or even suspended con-stituents and

• to recover a stream of water clean enough forpotential re-use.

It is clear that the studied process dependedprimarily on the nature, characteristics andefficiency of the applied bonding agents or met-als. For example, the particle size distribution ofthe bonding agents, their concentration and thesurface charge determine the flotation effective-ness. However, certain limitations were imposedon the examined parameters, for example airflow rate and the position of the membranes.The air flow rate affects the flotation process;experimental results published by variousresearchers indicate that flotation recoveryincreases to a maximum and then decreases asair flow rate increases (Laplante et al., 1983).On the other hand, the effectiveness of the mem-brane filtering was directly proportional to theair flow rate. Therefore, the hybrid cell operation

required an intermediate air flow rate in order toachieve the highest flotation recovery and at thesame time the lowest membrane fouling.

The effect of gas flow rate on membranepermeability for the simple MF system is pre-sented in Fig. 2. As the air flow rate (expressedas superficial velocity) was increased, the perme-ability of the membranes in simple MF systemswas increased. Injecting gas into the feed streamto create a gas–liquid two-phase cross flow cancontrol effectively cake formation in MF, thusenhancing the permeate flow rate. The influenceof bubbling in membrane applications dependson the relative dimensions of the flow path andthe bubbles. In the majority of cases bubblesbehave as slugs in close proximity to the mem-brane surface; these slugs create very high shearstresses on the membrane wall and improve theperformance of membranes (Cabassud et al.,1997, 2001). However, air slugs are not suitablefor flotation process. It is also remarkable thatthe gas sparging improved the MF operation indifferent levels for the different types of particles.

The induction of flotation conditions into themembrane reactor (addition of an appropriatesurfactant in the liquid dispersion) rendersthe solid particles hydrophobic. Thus, when the

Ferric flocs Zinc hydroxideHydrotalcite

8000

6000

4000

2000

0

UG = 0 cm s�1

UG = 0.265 cm s�1

UG = 0.447 cm s�1

UG = 0.647 cm s�1

Per

mea

bilit

y (L

m�

2 ba

r�1

h�1 )

Fig. 2. Effect of gas flow rate on permeability for the micro-filtration process; UL = 0.0168 cm s�1, [hydrotalcite] =2 g L�1, [Fe(III)] = 200 mg L�1, [Zn(II)] = 50 mg L�1.

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suspension is brought into contact with gas bub-bles, the solid particles collide with the ascend-ing bubbles; some of these collisions result inthe solid particles adhering to the bubbles, thusbeing removed to the surface of the three-phase system. Consequently, fewer solid par-ticles reach the surface and possibly the poresof the membrane, which results in less pro-nounced membrane fouling. The performanceof the hybrid flotation–MF process was com-pared to the performance of a simple MF systemand the results are shown in Fig. 3. The resultsindicate that the hybrid process has an enhancedperformance compared to the separate sub proc-esses. Over 550 min of operation, the membranewater permeability improved by 50–133% andby 75–200%, as compared to the simple MF sys-tem without and with air sparging, respectively.

During the experimental procedure, sampleswere obtained from the membrane outlet andthe bonding agent (zeolite) content was meas-ured. In all cases no zeolite was determined,obviously due to the efficient membrane filtra-tion. The metal (zinc) removal was more than95% for all the zeolite concentrations afterabout 2 h (Fig. 4) (Matis et al., 2004c). Thehybrid cell (volume 3300 cm3) was initiallyfilled with filtered tap water, the real suspensionfeeding flow rate was 27 cm3 min�1 and 2 h isthe essential time interval for the replacementof tap water by the suspension that containedthe remaining zinc ions and zeolite particles.

The effect of gas flow rate on the hybrid proc-ess (Fig. 5) is more complex, since air bubblesare involved not only in membrane surfacecleaning but also in the removal of solid particlesby flotation. In the hybrid flotation–MF experi-ments, the increase of air velocity affected themembrane performance in a different way.Using ferric flocs and zinc hydroxide as solidparticles, the increase of air velocity increasedthe hybrid cell performance, but up to a criticalvalue (uG = 0.447 cm s�1), beyond which anyadditional air velocity increase resulted in a

decrease of the cell performance. In the hybridprocess for zeolite separation (Peleka et al.,2006a), at low suspension feed flow rates theincrease of gas flow rate had little effect on per-meability. At higher suspension feed flow rates,however, membrane permeability was found todepend on gas flow: the higher the gas flowrate, the higher the TMP increase and the

1000

800

600

400

200

00

(a) Zeolite concentration (g L�1)

1 2 3 4 5 6

Per

mea

bilit

y (L

m�

2 ba

r�1

h�1 )

MF

MF � AIR

HYBRID

250

200

150

100

50

0

Zeolite concentration (g L�1)

1

(b)

2 3 4 5

Impr

ovem

ent o

f hyb

ridce

ll op

erat

ion

(%)

Comparison with MF system

Comparison with MF � system

Fig. 3. (a) Membrane permeability in microfiltration, micro-filtration with air sparging and hybrid microfiltration–flotation system versus solid concentration (after 550 minof operation); [Zn(II)] = 50 mg L�1, UG = 8.5 � 10�4 m s�1,UL = 6.7� 10�5 m s�1. (b) Improvement of hybridmicrofiltration–flotation system operation as comparedwith simple microfiltration system and microfiltration withair sparging system for various solid concentrations (after550 min of operation); [Zn(II)] = 50 mg L�1,UG = 8.5� 10�4 m s�1, UL = 6.7� 10�5 m s�1.

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corresponding permeability loss. In the hydrotal-cite case, membrane permeability was found tobe directly proportional to gas flow rate.

The solids concentration was one among theexamined parameters; the results are illustratedin Fig. 6. The higher solids concentration corre-sponds to a lower permeability. The effect ofsolids concentration was more evident in caseof zinc hydroxide and iron flocs may be due totheir colloidal nature. According to the type of

fouling materials, four categories of membranefouling are generally recognized:

1. inorganic fouling/scaling, by the accumula-tion of inorganic precipitates such as metalhydroxides, and ‘‘scales’’ on membrane sur-face or within pore structure;

2. particulate/colloid fouling by algae, bacteriaand certain natural organic matters fall intothe size range of particle and colloids; how-ever, they are different from inert particlesand colloids such as silts and clays;

3. microbial fouling is a result of formation ofbiofilms on membrane surfaces; once bacteriaattach to the membrane, they start to multipleand produce extracellular polymetric substan-ces to form a viscous, slimy, hydrated gel; and

4. organic fouling is found in membrane filtra-tion when water contains relatively highamounts of natural organic matters.

The cake, which was formed during the filtra-tion of iron colloidal particles and zinc hydrox-ide, was more compact than solid particles

100

90

80

70

60

50

Zeolite (g L�1)

10 2 3 4 5

Zn2

� r

emov

al (

%)

0 h

1 h

2 h

3 h

4 h

5 h

6 h

7 h

8 h

9 h

Fig. 4. Removal of metal (Zn2+ with initial concentra-tion 50 mg L�1) versus bonding agent (zeolite) concen-tration in the hybrid cell.

14000

12000

10000

8000

6000

4000

2000

0Hydrotalcite Ferric flocs Zinc hydroxideZeolite

Per

mea

bilit

y (L

m�

2 ba

r�1

h�1 )

UG = 0.265 cm s�1

UG = 0.447 cm s�1

UG = 0.647 cm s�1

Fig. 5. Effect of gas flow rate on permeability forthe hybrid flotation–microfiltration process; UL =0.0168 cm s�1, [zeolite] = 5 g L�1, [hydrotalcite] =2 g L�1, [Fe(III)] = 200 mg L�1, [Zn(II)] = 50 mg L�1.

14000

12000

10000

8000

6000

4000

2000

0

Per

mea

bilit

y (L

m�

2 ba

r�1

h�1 )

0

Solid concentration (g L�1)

1 2 3 4 5

Zeolite

Zinc hydroxide

Hydrotalcite

Ferric flocs

Fig. 6. Effect of solids concentration on permeability forthe hybrid microfiltration–flotation process; zeolite case:results after 9 h operation, UL = 6.7� 10�5 m s�1,UG = 8.5� 10�4 m s�1, zinc hydroxide case: results after5 h operation, UL = 1.68� 10�4 m s�1, UG = 4.47�10�3 m s�1, hydrotalcite case: results after 7 h operation,UL = 1.68� 10�4 m s�1, UG = 2.65� 10�3 m s�1, ferricflocs case: results after 6 h operation, UL = 1.68�10�4 m s�1, UG = 2.65� 10�3 m s�1.

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(zeolite and hydrotalcite) cake, and thus theinduction of flotation conditions had more effect.

Backflusing (BF) has been also applied, inorder to extend membrane operational time.The membrane backflushing process is carriedout by periodically reversing the direction of per-meate flow, and this inverse stream disrupts thecake deposited on the membrane surface andthe membrane pores. A backflush duration of5–20 sec and pulses 3–10 times per hour hasbeen recommended (Miculasek et al., 2000). Itsefficiency is quantified by a resistance index(RI) (Vigneswaran et al., 1996):

RI ¼ RF � RM

RO � RM

where RF is the resistance with backflush, R theresistance without backflush and RM the resist-ance with clean water. When RI approaches 1,it implies that backflush is inefficient. A smallerRI means efficient backflush, and a zero valuewould mean a perfectly efficient backflush.

The values of the RI for the separation of zeo-lite, hydrotalcite, zinc hydroxide and ferric flocs,at best backflushing conditions, are presented inTable 3. Backfushing is efficient in the case ofzeolite and hydrotalcite particles; in the case ofcolloidal particles (zinc hydroxide and ferricflocs), the data are not encouraging. It is prob-able that colloidal particles, which are depositedon the membrane surface, may require moredynamic conditions to be removed, but on theother hand such dynamic conditions would beincompatible with the flotation process. It isalso interesting to note that backflushing ismore efficient in the case of hydrotalcite par-ticles compared to zeolite particles; the meanparticle sizes of hydrotalcite and zeolite are15 mm and 3 mm, respectively, therefore back-flushing efficiency is found to increase, as theparticle size increases.

A feasibility study was also conducted (Lazar-idis et al., 2004) using an appropriate software for

technico-economic studies (SuperPro Designer,of Intelligen, Inc.). The capital investment forsuch a hybrid plant was estimated to be of theorder of 0.92 M⁄, while the operating costwould be about 1.42 M⁄ per year. The compari-son with a process involving the two processesseparately, that is MF followed by flotation,showed that the hybrid system has lower capitalcost and lower operating costs.

4. Conclusions

Membrane filtration technologies areincreasingly used for solid–liquid separationpurposes in conventional water and wastewatertreatment plants. One of the major problemsencountered during their operation is foulingby solid particles, causing a gradual degradationin process efficiency. It is possible to preventfouling by gas sparging which prevents solidparticles from depositing on the surface of themembranes. The bubble stream may be used atthe same time to remove the solid particlesfrom the dispersion by flotation into the frothlayer, with membranes being subjected to onlya part of the initial feed concentration of solidparticles.

The combination of the two processes into ahybrid process, with the membrane modulesubmerged inside the flotation cell, combinesthe advantages of both single processes. Theefficiency of the combined process is higher

Table 3

Resistance index at different backflushing conditions;

UL = 1.68� 10�4 m s�1, UG = 4.47� 10�3 m s�1

Suspensionconcentration(g L�1)

fBF (h�1) DtBF (s�1) RI (–)

Zeolite, 5 12 5 0.10Hydrotalcite, 2 12 10 0.05Ferric flocs, 0.2 12 10 0.83Zinc hydroxide, 0.05 12 5 0.42

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when compared to the single processes and otherproposed in the literature methods for preventingthe fouling of membranes. It also achieves usefulenergy savings since no additional energy thatneeded for flotation was necessary for counter-ing the fouling of the membranes.

References

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[9] Lazaridis, N.K., E.N. Peleka, Th.D. Karapantsios andK.A. Matis, Hydrometallurgy, 74 (2004) 149–156.

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