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A NEW HYBRID MEMBRANE – FLOTATION CELL FOR WATER TREATMENT AND

PURIFICATION

F. Peleka, P. Mavros, D. Zamboulis and K.A. Matis Chemical Technology and Industrial Chemistry Section

Department of Chemistry, Aristotle University GR-54124 Thessaloniki, Greece

Abstract

A new hybrid process for cleaning wastewater, combining membrane microfiltration and flotation, was investigated. The hybrid process combined the advantages of both membrane separation and flotation: the flotation cell removed a large proportion of suspended solid particles, while the membrane module produced a clean water permeate effluent. The proof of concept for the hybrid solid/liquid separation process was investigated using an aqueous suspension of ultra-fine zeolite particles. Process parameters investigated were: the effect of gas flow rate - which was necessary for the flotation process - and the suspension feeding rate. Preliminary results indicated that the increase of airflow rate affected positively the hybrid cell performance, up to a critical value, beyond which any additional increase resulted in a decrease of the cell performance. As for the suspension feeding rate, its increase led to higher transmembrane pressures and a higher total membrane resistance. Keywords: hybrid membrane, flotation, water treatment, purification. 1. INTRODUCTION

Fresh water is usually thought of being abundant and easily accessible; in reality, it seems that less and less water is available for the needs of an increasing global population, and a considerable disparity exists between the amounts available for the developed world and for the developing or under-

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developed countries. It has been estimated that 100 liters per person per day is the bare minimum required for basic household needs such as drinking, bathing, and cooking and that 5 to 20 times this amount is needed to meet the demands of the agricultural, industrial, and energy production sectors [1]. In Europe, 140 millions people live in regions where water is over-pumped, or near to urban centers where the water consumption rate exceeds the filling rate of underground and surface aquatic reserves. In 1995, the vice-president of the World Bank, emphasizing the water crisis, stated at a conference at Stockholm: “Many of the wars of this century happened for oil, but the wars of next century will happen for water” [2]. In addition to this potential shortage, another severe problem is the pollution of available fresh water streams (sources, resources) by various undesirable substances, some of which are toxic for humans and the fauna, e.g. metal ions, pesticides, etc. In some cases, these substances are unfortunately natural constituents and the pollution of these streams seems to have happened accidentally [3]. However, in most cases, the presence of these components is the result of some industrial or agricultural activity. National and international legislation enforces the treatment of these streams, in order to remove the noxious components, and various techniques have been developed to achieve this. In addition to more traditional water-cleaning techniques, like dissolved-air flotation (DAF), new processes are also used to obtain a water quality required for potable water. One such technique is membrane filtration [4] with the following advantages: ü constant quality of produced water ü considerably lower amount of chemicals used for the process ü easy process development (modular system) ü long operation periods without interruption for cleaning ü low energy consumption compared to other techniques

Membrane units, such as microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), have been widely used for pure water production. Today, more than 2 million m3/d of drinking water are being produced worldwide using low-pressure membranes, including MF and UF [5]. The attractiveness of using MF in conventional wastewater treatment is attributed to improved performance and reduced cost of membranes due to technological advances [7].

A hybrid process, combining membrane filtration and flotation, was applied to water cleaning from suspended solid particles. In this work a synthetic zeolite in a form of fine particles was used for the artificial suspension. Zeolite was chosen because it could serve as ion exchanger or adsorbent to treat aqueous effluents polluted with heavy metals. Attention

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was focused on the problem of membrane fouling, due to the presence of solid particles, and on the ways of countering this problem, in order to enhance the hybrid process performance.

2. MEMBRANE FOULING

The major obstacle in the extensive use of a microfiltration unit in

water and wastewater treatment is membrane fouling. Fouling is a generic term, which describes the gradual deterioration of membrane performance in terms of permeation flux and selectivity, due to accumulation of solids on their surface or inside their pores. The implications of fouling are: irreversible membrane damage, reduced flux rates or increased operation pressures and increased operating costs from frequent chemical cleaning [8]. The mechanisms of membrane fouling include pore blocking, concentration polarization and cake formation. When a membrane is first placed in an aqueous medium, a pore can be blocked partially or even completely by the retained particles because of the direct contact of the particles with the membrane pore surface. However, the mechanism of pore blocking may be neglected if the particles in the suspension are larger than the membrane pores.

Concentration polarization (CP) indicates the formation of a layer with a decreasing concentration gradient from the membrane side towards the bulk of the solution. This CP layer contributes to an additional resistance to the permeating flow. In microfiltration, the resistance of the CP layer is usually very small because of the large size of particles retained (> 0.1 μm).

When the rate of convective transport of the solute to the membrane is greater than the transport away from the membrane, a cake layer begins to form on the membrane surface, due to an increase of the particle concentration in the polarized layer on the membrane side, reaching a limiting concentration. The thickness of the cake layer increases as the amount of retained particles increases. The particle concentration of the cake layer is initially equal to the limiting concentration of the CP layer, but may increase over time if the cake compacts or finer particles in the permeating flow are also deposited on the cake. The formation of a cake layer adds a further resistance to the permeating flow. Cake formation is often the most important factor affecting the permeation flux of a MF unit [9]. The evolution of flux densities as a function of time for various operating conditions (transmembrane pressure, tangential velocity, tempe-rature and concentration of the solution/suspension) is a classic method of

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studying membrane fouling. Fouling causes a decreased permeability, which may be correlated to an increase in mass transfer resistance, and it modifies the selectivity of porous membranes. Each of these can be interpreted in terms of resistance [10]. However, the total hydraulic resistance appears in the Darcy – Poiseuille equation:

∆

=

totalRPAQ

µ (1)

where Q is the liquid flow rate [m3 s-1], A the surface area of membranes [m2], ΔP the transmembrane pressure [Pa], μ the dynamic viscosity of the suspension [N s m-2] and Rtotal the total hydraulic resistance [m-1].

Hydraulic, mechanical, chemical and/or electric methods are commonly used to clean fouled membranes. Membrane fouling can be minimized via feed pre-treatment [11], application of external fields (electric or ultrasound), backflushing [12, 13, 14], and/or gas sparging [14-18], amongst others. A combination of membrane filtration and air sparging processes integrated into one unit should minimize the defects of each system applied separately for solid particles removal. This combined process could be further improved, if instead of mere air sparging, a collector and / or frother is added to the suspension so that the most of solid particles are removed by flotation at the top of the cell.

A hybrid membrane filtration - flotation system, consisting of a membrane module submerged into a flotation column, was studied and results are reported here. A suspension of fine-sized zeolite particles was fed into the hybrid cell, where they were partially removed from the suspension by flotation, while clean water was drawn from the membrane module. Zeolite particles, which were not removed by flotation from the suspension, were deposited on the surface and the pores of the membrane, forming a cake that gradually blocked the pores and caused membrane fouling. The rising gas bubble swarm, while being used for the flotation process, at the same time was used as a membrane-surface scrubber, effectively preventing the solid particles from being deposited and also removing the deposited cake and in this way countering the fouling problem. 3. EXPERIMENTAL APPARATUS AND PROCEDURE

The hybrid cell consisted of a cylindrical flotation column made of plexiglass, (internal diameter (DC) 100 mm) and a flat ceramic porous gas

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sparger (average porosity Dpor = 40-100 μm, diameter Dsp = 60 mm), placed at 20 mm above the flat bottom of the column. A membrane module was positioned 60 mm above the gas sparger. The module consisted of a twin set of parallel, double-sided ceramic membranes, with a flat sheet multi-channel geometry. Figure 1a illustrates the scheme of the experimental rig and a schematic representation of the membrane module is shown in Figure 1b. The characteristics of the ceramic membrane were 0.3 μm mean pore size, total surface area 0.021 m2, hydrophilic surface properties and initial pure water flow rate capacity of about 2 L h-1m-2 bar-1.

(a) (b)

Figure 1. (a) Scheme of the experimental rig (1, mixing tank; 2, peristaltic pump; 3, air flow meter; 4, non-return valve; 5, slide valve; 6, manometer; 7, flotation column; 8, membrane microfiltration; 9, diffuser; 10, permeate collection tank; 11, foam collection tank). (b) Schematic representation of a module of the ceramic multi-channel flat-sheet membrane used in the tests.

The flow rate of the clean water permeate, which was drawn from the membrane module with a peristaltic pump, and the membrane pressure drop (TMP) were measured as a function of time for various experimental conditions. The pressure of the air, introduced into the cell through the sparger, was 1.1 bar.

A standard amount (5 g L-1) of synthetic type-A zeolite (Ineos Silicas Ltd, Warrington, Cheshire, UK), with a chemical formula Na2O.2SiO2.Al2O3.nH2O, was dispersed in water in a holding tank; the pH of the resulting dispersion varied from 10.0 to 10.5. The homogeneous

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suspension was continuously fed into the hybrid cell using a peristaltic pump. In the flotation experiments, cetyl trimethyl-ammonium bromide (CTMA–Br) was used as collector, to render the zeolite particles hydrophobic, condition necessary for an effective flotation; its concentration was kept constant at 10 mg L-1.

4. RESULTS AND DISCUSSION

All experiments were performed by feeding a 5 g L-1 zeolite suspension into the hybrid cell. The experiments were first conducted without air sparging and then with air sparging. In both cases no surfactant was used. The performance of the simple micro-filtration system, with and without air sparging, was compared to the performance of the hybrid microfiltration – flotation process. In Table 1 are presented the experimental conditions. Figure 2 illustrates the gradual change of the transmembrane pressure and the total membrane resistance versus operating time.

Table 1. Experimental conditions for the comparison between simple MF (with and without air sparging) and hybrid MF – flotation process. UL = 7.4×10-5 m s-1, xzeolite = 5 g L-1.

Process UG [m s-1] Collector [mg L-1] TMPinitial [bar] MF without air sparging (MF) 0 - 0.900 MF with air sparging (MF+Air) 8.5×10-4 - 0.875

Hybrid: MF+flotation 8.5×10-4 10 0.920

The presence of the rising gas bubbles slightly slowed down the membrane blockage, resulting in a slower total membrane resistance decrease. By adding the surfactant and inducing flotation conditions in the hybrid cell, a spectacular improvement of the cell performance was observed: for an operation lasting more than 500 min, the TMP dropped only 5% for the hybrid process, as compared to a 40% drop for the simple micro-filtration system. It became thus evident that the removal of the solid particles by flotation was beneficial to the overall process, since it reduced considerably the fouling of the membrane.

Further experiments were performed to investigate the effect of some operational characteristics, such as the liquid (UL) and gas (UG) flow rate – expressed here as superficial velocities, i.e., flow rates divided by the column cross-section area. Usually, flotation recovery increased to a maximum and then decreased as airflow rate increased [19]. On the other hand, the effectiveness of the membrane filtering was directly proportional to the airflow rate. Therefore, the hybrid cell operation required an intermediate airflow rate in order to achieve the highest flotation recovery and at the same time the lowest membrane fouling.

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0.6

0.7

0.8

0.9

1

0 120 240 360 480 600

MFMF+Airhybrid: MF+ flotation

RE

LATI

VE

TM

P [-

]

TIME [MIN] (a)

0

5 1011

1 1012

1.5 10 12

2 1012

0 120 240 360 480 600

MFMF+Airhybrid: MF + flotation

TOTA

L R

ES

ISTA

NC

E [m

-1]

TIME [MIN] (b)

Figure 2. Effect of air sparging on the evolution of the (a) relative transmembrane pressure and (b) total membrane resistance with time; xzeol = 5 g L-1, UL = 7.4×10-5 m s-1, UG = 8.5×10-4 m s-1 (where applicable).

Figure 3 illustrates the results obtained for various airflow rates. The

increase of air velocity affected positively the hybrid cell performance, but up to a critical value (UG = 0.447 cm s-1), beyond which any additional increase resulted to a decrease of the cell performance.

0.92

0.94

0.96

0.98

1

0 60 120 180 240 300 360

UG=0.085 cm/s

UG=0.265 cm/s

UG=0.447 cm/s

UG=0.667 cm/s

RE

LATI

VE

TM

P [-

]

TIME [MIN]

1 1011

2 1011

3 1011

4 1011

5 1011

0 0.2 0.4 0.6 0.8

TO

TAL

RES

IST

ANC

E [

m-1

]

UG[cm s-1]

(a) (b) Figure 3. (a) Effect of air superficial velocity (UG) on the evolution of relative transmembrane pressure and (b) influence of air superficial velocity (UG) on total membrane resistance (after 300 min of operation) for the hybrid microfiltration – flotation process; xzeolite = 5 g L-1, xsurfactant = 10 mg L-1, UL = 6.7×10-5 m s-1.

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The suspension feeding rate was also a determinant factor in the hybrid cell performance. In Figure 4 are illustrated the results obtained for various suspension flow rates: as the feeding rate increased, the suspension volume that was filtered per unit time went up and consequently the transmembrane pressure and the total membrane resistance were increased.

0.7

0.75

0.8

0.85

0.9

0.95

1

0 60 120 180 240 300 360

UL=0.0034 cm/s

UL=0.0067 cm/s

UL=0.101 cm/s

UL=0.0168 cm/s

RE

LATI

VE

TM

P [-

]

TIME [MIN] (a)

0

1 1011

2 1011

3 1011

4 1011

5 1011

0 0.005 0.01 0.015 0.02TO

TAL

RES

ISTA

NC

E [m

-1]

UL [cm s-1]

(b) Figure 4. (a) Effect of suspension feeding flow rate (UL) on the evolution of relative trans-membrane pressure and (b) influence of suspension feeding flow rate (UL) on total membrane resistance (after 240 min operation) for the hybrid micro-filtration – flotation process; xzeolite = 5 g L-1, xsurfactant = 10 mg L-1, UG = 4.5×10-3 m s-1.

5. CONCLUSIONS

Membranes are useful in producing clarified water from aqueous dispersions. However, fouling of the membrane surface and pores hampers their operation. In this work, it was demonstrated that it was possible to reduce substantially the fouling process by combining membrane micro-filtration with flotation. In such a process, flotation removed effectively the major part of solid particles, which were responsible for the fouling of the membrane. The comparison between the simple microfiltration process and the hybrid micro-filtration / flotation process showed that the combination of these two processes made it possible to extend the working time of the membrane, before a more vigorous cleaning process, like e.g. backflushing, was necessary.

The impact of system operation conditions on the performance of hybrid process, treating zeolite suspensions for pure water production was also examined. The gas flow rate and the suspension feeding rate were major operating variables and affected the efficient operation of the hybrid cell.

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The increase of air velocity affected positively the hybrid cell operation, but up to a critical value, beyond which any additional increase resulted in a decrease of the cell performance. The suspension feeding rate increase led to higher transmembrane pressures and total membrane resistances, as this implied a higher filtering volume per time unit. Acknowledgments Thanks are due to the Institute of Environmentally Compatible Process Technology (UPT) of the University of Saarland (Saarbrücken, Germany) for providing the membrane modules. The partial financial support of this work by the Greek Ministry of Education and the European Union is also gratefully acknowledged. References 1) Falkenmark M. and Widstrand C.: 1992, Population and Water

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16) Cabassud C., Laborie S., Durand-Bourlier L., J. Laine: 2001, Air sparging in ultrafiltration hollow fibers: relationship between flux enhancement, cake characteristics and hydrodynamic parameters. J. Membrane Science; 181:57-69.

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SYMPOSIUM FINAL PROGRAMME

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C. TZIMOPOULOS, C. EVANGELIDES, G. K. ARAMPATZIS & A. TSAOUSIS “Soil Water Balance in Horizontal Infiltration”

S.N. KAVALIERATOU, P.E. GEORGIOU, D.M. PAPAMICHAIL AND D.N. KARAMOUZIS “Alternative GIS Methods for the Derivation of Watershed Physiographic Information and its Impact on Synthetic Unit Hydrographs”

16:40-17:00 R.C. SCHULTZ, AYDIN TUFEKCIOGLU, T.M. ISENHART, J.P. COLLETTI, J.W.RAICH, W.W. SIMPKINS “Designing Riparian Buffers to Improve their Function”

G. GIULIANO, A.CANCELLIERE & G.ROSSI “Managing Uncertainty in Multi-Reservoir Operation through Adaptive Fuzzy Inference Systems”

S. BELIC & A. BELIC “Water Use Conflicts on the Land Reclamation Territory”

M. STEFOULI, D. DIMITRAKOPOULOS, J. PAPADIMITRAKIS& E. CHAROU “Monitoring and Assessing Internal Waters (Lakes) Using Operational Space Borne Data and Field Measurements”

17:00-17:20 A. PALA, P. GALIATSATOU, E. TOKAT, H. ERKAYA, C. ISRAILIDES & D. ARAPAGLOU “The Use of Activated Carbon From Olive Oil Mill Residue, for the Removal of Color from Textile Wastewater”

M.I.P. LIMA, D. SCHERTZER, S. LOVEJOY, J.L.M.P. DE LIMA & M.F.E.S. COELHO “Multifractal Tools Applied to the Study of Precipitation Extremes”

R. SAVIC, S. BELIC & A. BELIC “Reservoir Water Pollution in Agricultural Catchment and Priorities for Action”

E. TSIROS, C. DOMENIKIOTIS, M.SPILIOTOPOULAS & N.L. DALEZIOS “Use of NOAA/AVHRR Based Vegetation Condition Index (VCI) and Tmperature Condition Index (TCI) for Dought Monitoring in Thessaly, Greece

17:20-17:40 E. DOUKAKIS “Coastal Vulnerability and Risk Parameters”

N. O. BAYKAN & C. OZCELIK “Management of Drought”

G.METAXAS & G. KARANTOUNIAS “Investigation of the Use of Ikonos Data to Estimate the Irrigation Needs For Crops in a Medium Size Mediterranean Hydrological Basin”

A. MANTOGLOU & M. PAPANTONIOU “Optimal Design of Pumping Networks in Coastal Aquifers Using Evolutionary Algorithms”

20:00 ä å ä TURKISH-GREEK NIGHT ä å ä å

4 September 2004, Saturday 09:30-10:15 Hall B Dr. N. TSIOURTIS (CYP) “Risk and Challenges for the 21st Century” 10:15-11:00 Hall B Dr. U. OZIS (TUR) “Development of the Euphrates-Tigris Basin” 11:00-11:30 Coffee Break V. SESSION Hall A

THEME X: Recognizing and Valuing the Many Faces of

Water

Hall B

THEME VI: Mitigating Risk and Coping with Uncertainty

Hall C

THEME VIII & IX: Governing Water Wisely for Sustainable Development & Sharing Water: Defining a Common

Interest

Hall D

THEME XI: Modeling as a Tool in Decision-Making

11:30-11:50 H. ONDER & M. YILMAZ “Underground Dams: A Tool of Sustainable Development and Management of Groundwater Resources”

M.YILMAZ, Z. AKYUREK & N.USUL “A Methodology for Estiımating the Digital Elevation Model Uncertainty in Flood Inundation Modeling”

N. P. NIKOLAIDIS, N. SKOULIKIDIS & A. KARAGEORGIS “Pilot Implementation Of EU Policies in Acheloos River Basin and Coastal Zone, Greece”

G. K. SYLAIOS, V. A. TSIHRINTZIS & K. HARALA MBIDOU “Modeling Stratification-Mixing Processes at the Mouth of a Dam-Controlled River”

11:50-12:10 G. KARGAS, P. PATSIALOU & P. KERKIDES “Comparative Evaluation of Methodologies Used in the Determination of the Hydraulic Properties of Porous Media”

BIROL KAYA, RASOUL DANESHFARAZ, AHMET ALKAN & EBRU ERIS “Channel Capacity Analysis of Laka Stream for Floods”

E. SANER “EU Accession Process and Water Legislation Changes in Turkey”

G.P. KARATZAS K. KOLTSIDA & “Modeling the Groundwater Flow and Mass Transfer of Nitrates and Chlorides in the Pinios River Delta Aquifer”

12:10-12:30 E.KOUTSERIS “Methodologİes of Reconnaıssance and Evaluation of the Environmental Planning Impacts: For the Protection or Conservation”

I. BORDI, K.FRAEDRICH, M.PETITTA & A. SUTERA “Large-Scale Analysis of Drought in Europe Using NCEP/NCAR and ERA-40 Reanalysis Data Sets”

G. TSAKIRIS & M. SPILIOTIS “Multicriteria Ranking of Water Development Scenaria Using a Fuzzy Rule Based System

T. BARAN & U. BACANLI “Evaluation of Suitability Criteria in Stochastic Modeling”

12:30-14:00 ä å ä å ä LUNCH ä å ä å ä

VI. SESSION Hall A

Hall B

Hall C

THEME VIII & IX: Governing Water Wisely for Sustainable Development & Sharing Water: Defining a Common

Interest

Hall D

THEME XI: Modeling as a Tool in Decision-Making

14:00-14:20 W.O. OCHOLA, P. KERKIDES, J.O. AYIEKO “Assessing Wetland Conditions in the Lake Victoria Basin of Kenya Using Integrated Land-Use and Water Resources Integrity Indicators”

P.E. GEORGIOU, M. DIAMANTOPOULOU & D.M. PAPAMICHAIL “Modeling the Rainfall-Runoff Process Using the Slurp Watershed Model and Artificial Neural Networks”

14:20-14:40 M. RIAZ & Z. ŞEN “Aspects of Design and Benefits of Alternative Lining Systems”

G. BOMBAR & S. GUNEY “Study of Flows in Stepped Channels”

14:40-15:00 L. YILMAZ “Water Resources Management of Sapanca Lake”

H. BOYACIOGLU, H. BOYACIOGLU & O.GUNDUZ“Application of Factor Analysis in the Assessment of Surface Water Quality in Büyük Menderes Basin in Turkey”

15:00-15:20

GENERAL

ASSEMBLY OF EWRA

S. DAKOVA “Bulgaıian River’s Flow in a Changing Climate”

C. TZIMOPOULOS, I. HALKIDES, K. MOUTSOPOULOS & S. GIANNOPOULOS “Fuzzy Simulation of Soil Water Balance for the Case of Two Dimensional Unsteady Flow”

15:20-15:50 Coffee Break VII. SESSION Hall B 15:50-17:10 PANEL DISCUSSION ON WATER RESOURCES MANAGEMENT: RISKS AND CHALLENGES FOR THE 21st CENTURY

20:00 ä å ä å GALA DINNER ä å ä å ä