Journal of Membrane Science 228 (2004) 83–88

A hybrid MF process based on flotation

N.K. Lazaridisa, C. Blöcherb, J. Dordab, K.A. Matisa,∗a Section of Chemical Technology& Industrial Chemistry, School of Chemistry, Aristotle University, Box 116, Thessaloniki, Hellas GR-54124, Greece

b Institute for Environmentally Compatible Process Technology (upt) Ltd., Im Stadtwald Geb. 47, D-66123 Saarbrücken, Germany

Received 13 February 2003; accepted 15 July 2003

Abstract

In the present work, a two-stage process was used for zinc ions removal from aqueous solutions. The first stage consists of the sorption ofmetal ions onto zeolite and the second one the separation of the metal-loaded sorbent in a hybrid cell. The later combines dispersed-air flotationand micro-filtration in one unit. The main parameters investigated were zeolite concentration, solution pH, collector type and concentration aswell as the submerged membranes backflush. The higher the zeolite dose the higher the transmembrane pressure and the lower the permeabilitywas observed. Backflush found to have a small effect on hybrid cell operation, under the studied conditions, while the collector type wasa crucial parameter. Zinc ion removal was almost complete. A 90% zeolite recovery by flotation, which was the foreseen aim, reachedsuccessfully.© 2003 Elsevier B.V. All rights reserved.

Keywords:Membranes; Dispersed air; Zeolites; Metal ions; Industrial wastewater

1. Introduction

Integrated membrane filtration processes have been re-cently developed, usually combined in a bioreactor withimmersed membranes[1,2]. Compared with traditionalwastewater treatment processes, this type of membranebioreactors offers several advantages. Fouling control inthese is usually done by air bubbling, creating an upwardair/liquid flown and thus turbulence inside the modules[3].Air injection has been also used to improve filtration per-formance (flux, energy consumption) in crossflow filtration[4,5], in which it was also applied to a ceramic flat sheetmembrane[6]. In the latter, the membrane orientation wasevaluated. For given conditions of aeration, periodic back-washing (15 s every 5 min) gave an additional efficiencyby decreasing internal fouling. Bubbles have been found tobe efficient for limiting particle deposition and polarizationphenomena.

Fouling problems, very low membrane permeabilityand low water yield (up to 75%) are the main problemsencountered with conventional membrane processes (i.e.electrodialysis, nanofiltration and reverse osmosis) applied

∗ Corresponding author. Tel.:+30-2310-997743;fax: +30-2310-997759.

E-mail address:kamatis@chem.auth.gr (K.A. Matis).

for metal ions removal. High investment costs and period-ical membrane cleaning are entailed, while high quantitiesof wastewater are generated. On the other hand, selectiveadsorbents with fast reaction kinetics for binding the toxicmetals (like zinc) have been practiced for long time[7].

Scope of this paper constitutes the introduction of flota-tion in the same unit of operation with MF/UF separation,the impact of this hybrid cell being a new separation tech-nology [8]. In this way, the air bubbles being the necessarytransport medium of particulate matter for flotation wouldbe also and together used to the vacuum-driven membranesin their cleaning.Fig. 1 gives an idea of the integration ofthe process investigated into the overall process for heavymetals removal from wastewater.

For this reason, the dispersed-air flotation technique wasapplied for fine pore diffusion[9]. Flotation is a well-knownseparation method both in minerals processing as frothflotation, where it was originated, and nowadays in wa-ter and wastewater treatment[10]. Further, a pilot studysuggested that dissolved-air flotation (DAF) pretreatmenthelped reduce membrane fouling in hollow-fiber microfil-tration [11]. Membrane fouling by the DAF-pretreated wa-ter was characterized by reduced pore blockage and lowercake compressibility.

The use of a synthetic zeolite (of NaY type) for zinc ionremoval from dilute aqueous solution was published[12], in

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2003.07.024

84 N.K. Lazaridis et al. / Journal of Membrane Science 228 (2004) 83–88

selective

bondage

of metal ions

membrane

filtration

vacuum-driven hybrid process:

flotation combined with

submerged membranespurified water

for re-use or

discharge

higher contaminated

water with

toxic metal ions

regenerator/

separator

bonding agents

metal ions

concentrates or

solid metal for re-use

recycled

bonding agent

bonding and recycling stage separation stage

Fig. 1. The two-stage flow scheme for high concentrated (up to 500 mg l−1) metal ion removal[8].

a combined process later termed as sorptive flotation. Cer-tainly, what is worth noting, in this case it significant to payattention to chemical speciation[13]. For a number of years,zeolite A (4A) has been used in extensive tonnages through-out the world as a builder in detergents to replace phosphates;lately, the maximum aluminum p-type zeolite (MAP) wasintroduced, which has a more flexible structure[14].

2. Experimental

Zinc, used as a mere example of pollutant to be removed,was the metal ion under investigation. Its initial concen-tration in the feed solution was 50 mg l−1. The solutionwas kept agitated in a feed tank, of around 20 l content,in a continuous flow rig. Chemical analysis of the metalin the collected effluent samples was conducted by atomicabsorption spectrometry (AAS), in the standard manner. Inthe tank, with the help of mixing the zinc removal by ionexchange and precipitation was accomplished.

The zinc-bonding agent (adsorbent) used in the present,as a suitable model system for the separation, was a ze-olite of type 4A with chemical structure Na2O·2SiO2·Al2O3·nH2O—kindly supplied by Ineos Silicas. This ze-olite (designated as CA 150), according to the companyinformation, has an ion exchange capacity of 6 meq. g−1

and particle size (sedigraph) 3�m.The suspension was fed by a peristaltic pump to the con-

structed cell, at a superficial velocity of 7.4 × 10−5 m s−1

(liquid flowrate 0.58 × 10−6 m3 s−1), as imposed by theavailable membranes area. The duration of each experimen-tal cycle was 550 min, noting that the residence time in thecell was about 108 min; the respective flotation time was

only 10–15 min[12]. The cell was a perspex column withheight 0.48 m and inside diameter 0.10 m. The feed waterwas provided to the cell with or without the simultaneousair supply from the base, through a blower at around oneatmosphere excess pressure. The air superficial velocitywas 8.5 × 10−5 m s−1 (air flowrate 6.7 × 10−6 m3 s−1).

The arrangement was similar to the type used fordispersed-air flotation experiments, as normally[15]. Thediaphragm used for air bubbles generation, was from frit-ted glass (D4 type), with mean pore diameter 10–16�m.Ethanol was applied, if required (at pH 6) as frother, mean-while reducing the bubbles size.

Hexadecyl or cetyl trimethyl-ammonium bromide (de-noted as HDTMA) was the cationic collector applied in nat-ural pH, usually at the concentration of 10 mg l−1 and withconditioning time of 10 min. At pH 6, the anionic collec-tor sodium dodecyl-sulphate was used (60 mg l−1 SDS, plus0.15% v/v ethanol), as found appropriate. Recovery of ze-olites, denoted asRzeo (%), was calculated with turbiditymeasurements often coupled by gravimetric measurementsto close the material balance.

The membranes were coming from Hermsdorfer Insti-tut für Technische Keramik (Thuringia, Germany) and themodule was prepared in upt Ltd., Saarbrücken. The latter,with two vertical membranes was used here. These mem-branes were patented ceramic membranes with flat-sheetmulti-channel geometry and hydrophilic surface properties.The mean pore size was 0.3�m, the pure water flux about2 m3 m−2 h−1 bar−1. The membranes surface area was0.021 m2. The module was placed at around the one-sixthof the column height from the bottom, over the air diffuser.The permeate flux (provided by another peristaltic pumpat the exit) was 2.78 × 10−5 m3 m−2 s−1. Pressure was

N.K. Lazaridis et al. / Journal of Membrane Science 228 (2004) 83–88 85

measured by a transducer installed in the permeate line, forthe calculation of transmembrane pressure,Ptm. The ap-plied backflushing was normally every 30 min for 0.5 min,as found suitable.

3. Results and discussion

3.1. Metal sorption and loaded zeolites flotation

It is known that zeolite 4A exchanges its sodium ions bycalcium and magnesium ions in hard waters. In order to elu-cidate the mechanism of zinc ions uptake by zeolite batchwise sorption experiments were performed, for 20 min con-tact time. At the end of sorption the bulk, after filtration, wasanalyzed for sodium and zinc content. The following resultswere deduced: (i) employing 0.5 g l−1 zeolite, at pH= 6,ion exchange was responsible for 43% of zinc ions removal;and (ii) employing 2 g l−1 zeolite, at pH= 6, ion exchangewas responsible for 99% of zinc ions removal. However,the latter became about 83% in the presence of 50 mg l−1

of calcium ions; hence, a non-selective removal. Generally,the removal of the metal(s) was a matter of the amount ofbonding agent (zeolites) added.

To shed more light on zinc ion removal mechanism blanktests were performed versus pH. The following results wereobtained: (i) at pH= 6, zinc precipitation was almost nil;(ii) at pH 7 it was 51.4%; (iii) at pH 8 it was 94%; and (iv) atpH 9 it was 99.7%[13]. The natural pH of zeolite was foundaround 10.5. From the pH value of about 4.5 and towards theacidic region, it is noted that the zeolites were dissolving.

The two most common systems with dispersed air are sub-surface and mechanical; in the former, air is introduced inthe form of very small bubbles by diffusers or other devicessubmerged in the wastewater. Fine bubble aeration results inmore bubble surface area per unit volume, also greater num-ber of bubbles for the same air volume, but lower rise rate[15]. The size of bubbles that are released is a function ofthe pore diameter of the opening, liquid density and surfacetension. The specific role of bubbles size in flotation hasbeen examined; bubble coalescence was discussed too[16].Three methods of bubble generation in terms of average bub-ble diameter, bubble size distribution and power consumed,during production, were elsewhere published[17].

Table 1presents some selected results of batch flotation.The necessity of applying different flotation collector (either

Table 1Batch flotation results: [Zn] 50 mg l−1

[Zeolite] (g l−1) pH [Collector] (mg l−1) Rzeo (%)a

2 6 [SDS]= 20 972 6 [SDS]= 40 >992 10.5 [HDTMA] = 10 712 10.5 [HDTMA] = 20 >96

a Batch flotation.

SDS or HDTMA) depending on the solution pH was noticed.It was also observed, from the chemical analysis of surfac-tants, that as flotation time was increased, the remaining inthe solution collector was decreasing, being transferred bythe air bubbles towards the liquid surface.

Another finding had to do with the influence of zeoliteaging in solution or wetting. At pH 6, a drop of flotationrecovery with time was observed. Zeolites contain largeamounts of water, at ambient conditions; upon dehydra-tion, they undergo considerable structural changes, such aspore shrinkage, cation migration, and even total collapse[18]. In this pH case, as a possible counteract, the advan-tageous use of a polyelectrolyte (2 mg l−1 Zetag 87) wasfound.

In this observation, a further assistance was given by anelectrokinetic insight, in the laboratory. The measurementsshowed an immediate iso-electric point (i.e.p.) of pH 8;which, however, was moved to about 5.75 when the zeo-lites were well wet. At acidic pH values under the i.e.p.,the solid had a positive surface charge, being changed toa negative charge when overpassing that value, moving to-wards basicity. This study also helped in the decision ofthe suitable flotation collector, according to the conditions.The ζ-potential was shifting towards the basic or acidic re-gion, when respectively the cationic or anionic surfactant

0 120 240 360 480 600-500

-400

-300

-200

-100

0

without air without flotation with flotation

Ptm

(mba

r)

time (min)

0 120 240 360 480 6000

200

400

600

800

1000

1200

1400 without air without flotation with flotation

perm

eabi

lity

[L/m

2 hbar

]

time (min)

(a)

(b)

Fig. 2. Influence of air and flotation on the time variation of (a)transmembrane pressure and (b) permeability. Experimental conditions:[Zn] = 50 mg l−1, [zeolite]= 5 g l−1, [HDTMA] = 10 mg l−1, pH = 10.5.

86 N.K. Lazaridis et al. / Journal of Membrane Science 228 (2004) 83–88

was added. It is also known that flotation should be workingat optimum conditions[10,15].

So, in the continuous flow experiments (of around 9 hoperation), given in details in the next chapter, with 1 g l−1

zeolites in the feed, the zinc removal was 95% at naturalpH and only 85% at pH 6. While, with 3 g l−1 zeolites, zincremoval was 97% at natural pH and 90% at pH 6.

3.2. The hybrid flotation-MF cell

It was earlier found that the more fine were the air bub-bles applied, experienced by inserting in the solution afrother (ethanol) or changing the mean pore diameter ofthe air diffuser used (D2 instead of D4), the lower was thepressure drop through the membranes. So, this led to theuse of the said sparger, and also the addition of a surfactantthat anyway is required to undergo flotation. The likely ex-planation to this finding is that the flow with finer bubblesis more uniform, which means that the small bubbles aremore likely to be effective at the surface of the membrane.With bigger bubbles, at the same flow rate, it is possi-ble that the main bubble flow passes in a greater distancefrom the membrane; thus, creating lower effective shearrate.

0 120 240 360 480 600-500

-400

-300

-200

-100

0

without air without flotation with flotation

Ptm

(mba

r)

time (min)

0 120 240 360 480 6000

200

400

600

800

1000

1200

1400

without air without flotation with flotation pe

rmea

bilit

y [L

/m2 hb

ar]

time (min)

(a)

(b)

Fig. 3. Influence of air and flotation on the time variation of (a)transmembrane pressure and (b) permeability. Experimental conditions:[Zn] = 50 mg l−1, [zeolite]= 3 g l−1, [HDTMA] = 10 mg l−1, pH = 10.5.

0 120 240 360 480 600-400

-300

-200

-100

0

without air without flotation with flotation

Ptm

(mba

r)

time (min)

0 120 240 360 480 6000

200

400

600

800

1000

1200

1400

without air without flotation with flotation

per

mea

bili

ty [

L/m

2 hb

ar]

time (min)

(a)

(b)

Fig. 4. Effect of air and flotation on the time variation of (a) transmembranepressure and (b) permeability. Experimental conditions: [Zn]= 50 mg l−1,[zeolite] = 1 g l−1, [HDTMA] = 10 mg l−1, pH = 10.5.

The air presence, in other words, improved the membranesoperation.Figs. 2–4present some of the obtained resultswith the operation cell, giving good behavior. Lower trans-membrane pressures and higher permeabilities were foundwith air flow; at the greater solids concentration this effectwas more apparent and particularly, during flotation. In anycase, there were no zeolites passing through the membranes.

Similar results were found for the other solids concen-tration tested: for example, with 4 g l−1 zeolites, thePtmwas−120 mbar for the hybrid operation,−250 mbar for airand 0.1% ethanol present (no flotation),−280 mbar for airsparging alone, and−350 mbar for only microfiltration. Therespective permeability figures were 800, 400, 363.6 and285.7 l m−2 h−1 bar−1.

From the figures, it can be concluded that membraneperformance is to a higher extent influenced by the concen-tration of zeolites in the bulk solution (related to flotationefficiency) than by the presence of air. The relative increasein membrane permeability by introducing air is about 20%,whereas, if this air is used for flotation, permeability isnearly doubled.

Then, the effect of the collector was investigated at nat-ural pH (Fig. 5). With 10 mg l−1 HDTMA, the Ptm was−140 mbar and the permeability 714.3 l m−2 h−1 bar−1.The respective values at the adding of 20 mg l−1 HDTMAwere−120 mbar and 800 l m−2 h−1 bar−1—in other words,

N.K. Lazaridis et al. / Journal of Membrane Science 228 (2004) 83–88 87

0 120 240 360 480 600-200

-150

-100

-50

0

20 mg L-1 HDTMA

5 mg L-1 HDTMA

Ptm

(mba

r)

time (min)

0 120 240 360 480 6000

200

400

600

800

1000

1200

1400

20 mg L-1 HDTMA

5 mg L-1 HDTMA

perm

eabi

lity

[L/m

2 hbar

]

time (min)

(a)

(b)

Fig. 5. Effect of collector concentration on the time variation of (a)transmembrane pressure and (b) permeability. Experimental conditions:[Zn] = 50 mg l−1, [zeolite]= 1 g l−1, pH = 10.5.

improved. However, in the latter case difficulties were ex-perienced to control frothing; hence, the used collector con-centration was excessive. With 5 mg l−1 HDTMA, the Ptmand the permeability were a little lowered in comparison to20 mg l−1.

Table 2shows the comparison with the batch flotation re-sults alone, presenting may be a “surprise”; which is ratherusual, steady state flotation results to improvement. Themembranes forming a 100% barrier for the zeolites can ex-

Table 2Comparison between continuous flotation and hybrid system

[Zeolite] (g l−1) pH [Collector] (mg l−1) Rzeo (%)

Continuous flotation Hybrid without BFa Hybrid with BFa

1 10.5 [HDTA] = 10 85 892 10.5 [HDTA] = 10 85 933 10.5 [HDTA] = 10 78 904 10.5 [HDTA] = 10 82 93 975 10.5 [HDTA] = 10 80 94 971 10.5 [HDTA] = 5 – 82 –1 10.5 [HDTA] = 20 – 89 –1 6 [SDS]= 40 – 66 773 6 [SDS]= 60 – 74 –

a BF: backflushing.

0 100 200 300 400 500 600-200

-150

-100

-50

0

time (min)

Ptm

(mba

r)

with backflushingwithout backflushing

0

200

400

600

800

1000

1200

perm

eabi

lity

[L/m

2 hbar

]

with without

Fig. 6. Effect of backflushing on the time variation of transmembranepressure and permeability. Experimental conditions: [Zn]= 50 mg l−1,[zeolite] = 4 g l−1, pH = 10.5, [HDTMA] = 10 mg l−1.

plain the results. For a batch experiment the zeolites thatare not floated to the surface within the 10–15 min flotationtime are not recovered in the froth but remain in the bulk so-lution. In contrast, in membrane flotation these zeolites areretained by the membranes and are thus likely to be trappedby the bubbles or other particles later on and thus finallyalso transferred to the froth.

The influence of backflushing was also examined. The cellwas working better than the “simple” system (without back-flushing), as shown inFig. 6. Again this can be explained bythe membrane operation, as with backflushing the zeolitesthat form a cake layer on the membrane surface, are forcedback into the liquid and can be transferred to the froth, thus,increasing recovery.

Nevertheless, the process at pH 6, even with varied collec-tor, was seen quite problematic (Fig. 7), the possible reasonswere explained in the aforementioned.Table 2presents fur-ther comparisons, being in favor at the natural pH (i.e. giv-ing higher recoveries, greater feed solids concentration, lessof collector). The obtained recovery of 97% is noteworthy.

In wastewater treatment processes, generally, aeration in-troduces air into a liquid providing an aerobic medium formicrobial degradation of organic matter[9]. In the present,it may be said that a different application of bubble aera-

88 N.K. Lazaridis et al. / Journal of Membrane Science 228 (2004) 83–88

0 100 200 300 400 500 600-200

-150

-100

-50

0

time (min)

Ptm

(mba

r)

with backflushing without backflushing

0

200

400

600

800

1000

1200

perm

eabi

lity

[L/m

2 hbar

]

with without

Fig. 7. Effect of backflushing on the time variation of transmembranepressure and permeability. Experimental conditions: [Zn]= 50 mg l−1,[zeolite] = 3 g l−1, [SDS]= 40 mg l−1, pH = 6.

tion has been investigated. Certain limitations were furtherimposed on the parameters examined, like the air flow,bubbles size and apparatus design. It is necessary to usethe generated bubbles in flotation and membranes cleaningsimultaneously.

4. Conclusions

The hybrid cell is now a reality. The main conclusions ofthis work are the following

(a) There was an influence of solids concentration, as ex-pected.

(b) Frothing, with the appropriate dosage of surfactantand/or frother, should be controlled. Backflushing (gen-tle) contribution had no important influence and ratherimproved the operation.

(c) The results were better at natural pH, where flotationshould be operated. The cell’s efficiency, expressed asflotation recovery, was found better than the respectivebatch flotation results.

The hybrid cell was observed in the laboratory to workwell: the combination of flotation—membranes filtrationwas proved satisfying (recovery of∼90%). This means au-tomatically that, in a hypothetical case, if has to separatea feed with 4 g l−1 solid particles (here, the metal bondingagent), following flotation the actual system, treated by themembranes will only have the concentration 0.4 g l−1; withall its fouling effects onto membranes. It is needless to addthe apparent advantage of having a compact volume of thehybrid cell and that no energy was necessary for the foulingcontrol, in addition to that needed anyway for the flotationseparation.

Acknowledgements

This research comprises part from a program funded byEU, with contract no. EVK1-CT-2000-00083. Many thanks

are due to the coordinator Dr. V. Mavrov (Saarland Univer-sity) for his help; also, to Dr. M. Webb (ex-Crosfield) forthe zeolites sample and relative information, Ms. E. Peleka(AUTh) for experimental collaboration and Prof. M. Dohnal(TU Brno) for useful discussion.

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