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Development of bubble-less ozonation and membrane filtration process for the treatment of contaminated water
Journal of Membrane Science 492 (2015) 40–47
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Development of bubble-less ozonation and membrane filtration process for the treatment of contaminated water S.K. Stylianou a, S.D. Sklari b, D. Zamboulis a, V.T. Zaspalis b,c, A.I. Zouboulis a,n a
Section of Chemical Technology, School of Chemistry, Aristotle University, GR-54124 Thessaloniki, Greece Chemical Process and Energy Resources Institute, Center for Research and Technology Hellas, GR-57001 Thessaloniki, Greece c Department of Chemical Engineering, Aristotle University, GR-54124 Thessaloniki, Greece b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 18 May 2015 Accepted 20 May 2015 Available online 29 May 2015
The aim of this research was the evaluation of performance efficiency of a continuous treatment process, integrating bubbles-less ozonation and membrane filtration for water purification. A new bench-scale experimental apparatus was designed, comprising of two ceramic membrane modules, connected in series. In the first module, bubble-free ozone contacting takes place via a modified α-Al2O3 membrane. Chemical modification of a-Al2O3 membranes was performed with four different modifying solutions in order to change their hydrophilic behavior and thus, to reduce membrane resistance to ozone transfer. The hydrophobic character of initial and modified membranes was examined by contact angle measurements. The results showed that the chemical modification of membranes decreased the hydrophilic character of the original membranes and in certain cases transform them to hydrophobic. The values of measured contact angles were between 761 and 1431. In the second treatment module (in series), a microfiltration or ultrafiltration ceramic membrane was placed in order to separate suspended particles, macromolecules and/or arsenic. The hybrid process was evaluated for the treatment of aqueous solutions, contaminated by humic acid, kaolin (clay) and/or arsenic. The results indicated that the combination of ozone with membrane filtration limits fouling (around 25%) and improves the removal of arsenic (up to 80% removal), although it had no effect on TOC reduction. & 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrophobized α-Al2O3 membranes Ozone membrane contactor Hybrid membrane processes Membrane fouling Arsenic
1. Introduction The degradation of water quality is a matter of major concern worldwide. In recent years, the construction of central water/wastewater treatment plants has provided a solution to conventional water pollution problems; however, there are several contamination cases, i. e. occurrence of heavy metals, refractory organic pollutants, etc., requiring specific/supplementary treatment, before the treated water can be used or reused [1–3]. Membrane filtration systems, which represent a relatively new and promising technology in this field, could deliver flexible and efficient solutions for smaller scale in-situ treatment, especially for slightly contaminated water. The major drawback for the wide scale implementation of membrane processes, regarding water treatment, is membrane fouling, which results to the deterioration of membrane operation, the need for fouling control and consequently, to the increase of water treatment cost [4]. Several methods have been proposed in order to address and handle membrane fouling, including physical and chemical cleaning [5], membrane surface modification [6], appropriate
n
Corresponding author. Tel./fax: þ 30 2310997794. E-mail address: [email protected] (A.I. Zouboulis).
http://dx.doi.org/10.1016/j.memsci.2015.05.036 0376-7388/& 2015 Elsevier B.V. All rights reserved.
feed water pretreatment [7], etc. An alternative approach for the mitigation of membrane fouling, as well as for the improvement of permeate quality is the implementation of hybrid membrane processes, where membrane filtration is combined with another (water) treatment method in a concurrent mode. As a result, several studies have presented the combination of membrane filtration with other water treatment processes, such as flotation [8], coagulation [9], adsorption [10], oxidation [11] etc. Ozonation was successfully applied in several applications of water/wastewater treatment field, such as in disinfection, in oxidation of refractory organic compounds, of color, of odor and taste removal etc. [12]. Ozonation in a hybrid membrane process is expected to reduce the membrane fouling rate and further to improve the quality and the biodegradability of permeate. Several studies have been published on the effect of hybrid ozonation– membrane filtration processes on fouling [13], as well as on bromated formation [14], on herbicides oxidation [15], on the formation of disinfection by-products [16] etc. In most cases, ozone transfer to the water to be treated was carried out via injectors, bubble columns and gas diffusers. Common feature of all these techniques is the formation of bubbles; however in these cases, gas-liquid transport is a rate limiting process, depending upon the ozone bubbles diameter. Such a process never results in a
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complete ozone reaction, even with smaller gas bubbles; additionally, the ozone contactor suffers from foam problems, while an ozone (mostly thermal) destructor is usually connected to the device for the post-treatment (removal) of non-reacted gas. An alternative approach is the use of appropriate membrane contactors in order to optimize the contact between the gaseous and aqueous phase and to achieve almost complete (stoichiometric) ozone consumption [17]. However, in that case only chemically inert membranes can be used, such as ceramics, due to ozone's high reactivity with the most polymeric ones. Ceramic membranes offer high chemical, thermal and mechanical stability and porosity structure and sufficient flux. The main inorganic membranes available are made from metal oxides, e.g. alumina, titania, zirconia and silica. These metal oxides are characterized by hydrophilic behavior, due to the presence of hydroxyl (OH–) groups on their surfaces. Water with contact to a hydrophilic membrane can penetrate into the pores, due to capillary forces. The level of capillary pressure in hydrophilic porous media, as foreseen from theoretical approaches, is a function of pore diameter and can reach very high values, creating problems in gas transfer through it [18]. Inversely, the suitability of hydrophobic membranes, applied for the efficient contact between gaseous and aqueous phases has been confirmed by the respective mass transfer calculations [19,20]. Kukuzaki et al. calculated that the mass transfer coefficients for hydrophbized SPG membranes were four order of magnitude higher, when compared to hydrophilic ones [21]. The hydrophobicity of ceramic membranes can be achieved through chemical modification, surface morphology modification, or by the combination of them. Hydrophobized ceramic membranes are usually prepared via chemical modification and especially, by using organosilanes as surface modifying agents [22]. In this case, several organosilanes can be used, as for example, chloroalkylsilanes, fluoroalkylsilanes (FAS), polydimethylsiloxane (PDMS), triethoxy-1H,1H,2H,2H-perfluorodecyltriethoxysilane [23–25]. The aim of this study was the development of a hybrid process, combining bubble-less ozonation and membrane filtration for water treatment. It is considered as a novel approach of a hybrid process and the relevant published studies on its performance efficiency are rather scarce [26]. The use of membrane contactor for ozonation enables rapid ozone mass transfer and uniform reaction with pollutants, avoiding the need for high ozone fluxes. Commercially available tubular α-Al2O3 membranes were chemically modified by four different hydrophobic polymers and their wetting properties were examined. Hydrophobized membranes were used as membranes contactors for the transfer of ozone to the contaminated water via diffusion phenomena (i.e. without dispersion or mixing to take place). Microfiltration or ultrafiltration alumina membranes were used as a subsequent processing separation step. Humic acids were selected as common representatives of natural organic matter in waters, creating severe fouling problems to membranes, while kaolin (clay) was used as a common source of turbidity. In addition, arsenic was selected as refractory inorganic pollutant that has been identified in alarming high concentrations in several regions around the world. The study was focused on the advantages of using a hydrophobic ceramic membrane contactor for bubble-less ozone transfer and on the effects of the hybrid process to membrane fouling and to TOC, UV254 and arsenic removals.
2. Experimental 2.1. Surface modification of contacting.
α-alumina membranes for ozone
Commercially available tubular and planar alumina membranes were used for evaluating the effectiveness of the different surface modification solutions. The tubular ceramic substrate consists of
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three α-Al2O3 layers with a pore diameter of 900 10-9 m (base support), 500 10 9 m (intermediate layer) and 100 10-9 m (microfiltration layer), respectively. This specimen has length of 340 10 3 m, inner diameter (ID) 8 10 3 m, outer diameter (OD) 14 10 3 m, and surface 8.5 10 3 m2. The α-Al2O3 planar membrane has 50 10 3 m diameter, 80 10 9 m pore diameter and 2 10 3 m thickness; were also modified only for evaluation and comparison of their hydrophobic character in relation to the respectively modified tubular membranes. Four different hydrophobic polymers were investigated in this work for modifying the surface of ceramic membranes; 1,2dipalmitoyl-sn-glycero-3-phosphocholine (C40H80NO8P, 99% purity), trichloromethylsilane (CH3SiCl3, 97% purity), methyldiethoxysilane (C5H14O2Si, 96% purity) and octadecyltrimethoxysilane (CH3(CH2)17Si(OCH3)3, 90% purity) [27–29]. The precursors were obtained from Sigma-Aldrich and their chemical structures are illustrated in Fig. S1 (Supporting information). The hydrophobic functionalization of internal surface layer of the tubular alumina membranes was achieved via a slip casting technique, by applying a contact time of 20 s between the modification solutions and the membrane surface, by using the experimental setup described in Fig. S2 (Supporting information). The solution of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was prepared in chloroform at a concentration of 50 mg/L . 200 ml of NaCl 10 3 M was added in the film that was formed after the evaporation of CHCl3 and the new solution was passed through the internal surface of tubular membranes. The tubes were dried at room temperature overnight through evaporation in order to remove the small quality of remaining solvent, from the pores of tubular alumina membranes. The modified membranes via 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine were labeled as “DPPC”. The solution of trichloromethylsilane was prepared by dissolving 2.36 mL of the polymer in 150 mL toluene; after the membranes removed from the solution, they were rinsed five times with ethanol to remove any unreacted chemicals. The modified membranes were calcinated at 200 1C for 1 h in pure N2 atmosphere at heating rate of 0.5 1C min 1. The modified membranes via trichloromethylsilane were referred to as “triClMS”. The silanization of α-Al2O3 tubular ceramic membranes was also performed by a 95% ethanol solution, containing a 2 wt% addition of methyldiethoxysilane or octadecyltrimethoxysilane. The ethanolic solution of the silanization reagent was acidified with acetic acid to a pH of 3.3 or 4.5, respectively. The tubes were then rinsed five times with ethanol and were allowed to dry at room temperature. To complete the hydrolysis of silanization reagents, the tubes were placed in an oven at 110 1C for 30 min with heating rate of 1 1C min 1. The membranes modified via methyldiethoxysilane or octadecyltrimethoxysilane were named “MDS” or “ODtriMS”, respectively. The same procedure for hydrophobic modification was used for the case of planar membranes. These membranes were completely immersed in the same solutions as the aforementioned tubular membranes for 20 s. 2.2. Contact angle measurements. Contact angle measurements in the case of tubular ceramic membranes are more complicated, than for the planar membranes. The standard procedure for this measurement is to put a drop on the top layer of membrane and record it with a special camera. For the tubular ceramic membranes the top layer is located inside the tube. Unfortunately, this procedure causes damage in the membrane structure and the membrane cannot be used for any other purpose. For that reason, only the modified planar membranes presenting the highest hydrophobicity were selected, so as to prepare the respective tubular membranes for the measurements of contact angle.
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2.2.1. Planar membranes The macroscopic apparent contact angles of ultrapure water (resistivity at 25 1C: 18 MΩ cm) on the membrane surface were determined by the sessile drop method, using a drop shape analysis instrument (CAM 200, KSV). The measurements were performed by using 4.070.2 μL droplets. Static apparent contact angles were determined for the hydrophobic surfaces, whereas for the hydrophilic surfaces the contact angle was measured at the time of contact (o500 ms). However, the contact angle for the hydrophilic membrane surface changed drastically with time and such a contact angle measurement is not accurate. For that reason the time required for the droplet to penetrate the porous medium can be considered only as a semi-quantitative indication of differences in wettability between the different membranes [30]; the measurements were repeated 3 to 10 times and the average value with the respective standard deviation is presented in the figures. 2.2.2. Tubular membranes The contact angle of tubular membranes was measured, according to the static sessile drop method. The membranes were cut into pieces of 1 10-2 m and each piece was cut in quadrants. On each piece the contact angle was measured by placing a droplet on the membrane surface and the contact angle was measured by a high resolution camera (Data Physics Contact Angle System OCA 15 plus). A software system can analyze the angle formed between the liquid/solid and the solid/vapor interface and calculate the contact angle. For the value of the contact angle the first stable image, which it was taken was considered to represent the contact angle; with this procedure the evaporation of the water droplet was excluded, as well as the capillary intrusion into the porous material. 2.3. Experimental unit A schematic representation of the bubble-less ozonation membrane filtration experimental set up is shown in Fig. 1. It consists from two ceramic membrane modules, connected in series. Appropriate plexiglas vessels were constructed in order to house the membranes and the measuring devices. In the first module, bubble-free ozone contact (and oxidation) takes place via the modified hydrophobic aAl2O3 membrane, while at the second module, microfiltration or ultrafiltration membranes were placed in order to separate suspended particles, macromolecules (of natural organic matter, NOM) and/or metal ions. Ozone–oxygen gas mixture was produced by an ozone generator (model TOGC2A, Ozonia Triogen), where pure oxygen was used as a feed gas at a pressure of 0.05 MPa. The pressure of ozone–oxygen gas mixture produced by the ozonator was monitored by a digital pressure meter (WIKA, model S-10), whereas the flow rate of ozone–oxygen gas mixture was measured and adjusted by a flow meter, equipped with a needle valve (Aalborg, model PMR-1). Ozone– oxygen gas mixture was transferred from the outer to the inner surface of the hydrophobic a-Al2O3 membrane and then to contaminated water. A peristaltic pump (Watson Marlow, model 503U) was used to drive the contaminated water within the membrane at atmospheric pressure and at various flow rates. The calculation of ozone concentration in the influent gas mixture was taken place by passing the corresponding gas stream through a 2% KI trap, following the determination of ozone in the solution, according to the Standard Methods [31]. The residual (after treatment) dissolved ozone concentration was measured by an ozone sensor (ProMinent, type OZE). Two different membranes were used in the second module, where membrane filtration was taken place, aiming to the separation of humic acids fragments, etc., ozone oxidation products, kaolin and/or arsenic ions. An (unmodified) microfiltration a-Al2O3 membrane with an average pore diameter of 100 10 9 m (around 1000 kDa MWCO)
and an ultrafiltration γ-Al2O3 membrane with an average pore diameter of 3–5 10 9 m (around 50 kDa MWCO) were used. The γ-Al2O3 membrane was prepared via dip coating technique, as described in a previous study [32]. The ozonated water was driven through the membrane by the pressure created using an inox progressive cavity pump (Syden, model BM005-4S). TransMembrane Pressures (TMPs) were measured by a digital pressure meter (WIKA, model S-10). Permeate flux was continuously monitored by a mass balance (Kern, model FCB 8K01) (Fig. 1). 2.4. Feed water solutions/analytical methods. Two different aqueous solutions were used for the evaluation of performance efficiency of the hybrid process. For all experiments the pH and temperature of the feed solutions were adjusted at 7 and 20 1C, respectively. A synthetic solution containing 25 mg/L of humic acids and 25 mg/L of clay particles was prepared by dissolving humic acid stock solution and dispersing clay particles in the tap water of Thessaloniki city in order to simulate a medium contaminated surface water. Information about the respective concentration of major parameters in tap water of Thessaloniki is given in Table S1 (Supporting information). Humic acid (sodium salt) was obtained from Aldrich Chemical Company, while clay powder was commercially obtained. Initial samples and permeates were analyzed for the determination of the following parameters: UV absorbance at 254 nm (used as an indication of unsaturated organics concentration) and UV absorbance at 436 nm (used as an indication of color) were measured by a Hitachi UV–vis spectrophotometer; Total Organic Carbon (TOC) by a TOC-VCSH Total Organic Carbon Analyzer (Shimadzu); turbidity by a Hach Ratio/XR turbidity meter. The initial turbidity of treated water was 21.5 NTU, while the Total Organic Carbon (TOC) was 8.5 mg/L. Groundwater was collected from a drilling well, located in the settlement of Arachos in the municipality of Alexadria (outside of Thessaloniki) and it was used as the other feed water. This groundwater been found to contain a relatively high concentration of arsenic, up to 44 μg/L. Total arsenic concentrations for the initial samples and for the permeates were measured by graphite atomic absorption furnace, using PerkinElmer AAnalyst 800. In this case the initial turbidity of water was 0.4 NTU, while the Total Organic Carbon (TOC) was 0.55 mg/L. 2.5. Single ozonation experiments In order to select the optimum operating conditions for the first ceramic membrane module, single ozonation experiments were conducted, without the use of the second microfiltration/ultrafiltration module. Experiments were performed with a constant ozone flow rate of 0.03 L/min, corresponding to ozone mass flow rates of 0.7 mg/min. The low ozone flow rate was selected in order to avoid the formation of bubbles, which was among the key objectives of this study, noting that in higher ozone flow rates, the creation of rather large bubbles was observed. The feed water solutions were prepared in the influent tank, subsequently the water to be treated was driven by a peristaltic pump to the inner surface of the ceramic membrane, where it was contacted with ozone. Different inlet ozone concentrations were applied, by changing the liquid phase velocity and consequently, different contact times between ozone and contaminated water can evaluated. 2.6. Membrane filtration with simultaneous use of ozonation. After the completion of single ozonation experiments and the selection of optimum operating conditions of the first membrane
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Fig. 1. Flow chart of the hybrid unit of ozonation and membrane filtration used for contaminated water treatment. (1) O2 tank, (2) Ozone generator, (3) On-off valve, (4) Needle valve, (5) Ozone flowmeter, (6) Influent tank, (7) Peristaltic pump, (8) Water flowmeter, (9) Ozonation module, (10) Manometer, (11) Neddle valve, (12) Ozone trap, (13) Ozone analyzer, (14) Progressive Cavity pump, (15) Manometer, (16) Membrane filtration module, (17) Regulator Valve, and (18) Mass Balance.
module, additional experiments with the simultaneous use of ozonation and membrane filtration were conducted. The results obtained from the hybrid unit were compared also with the preliminary single membrane filtration tests in order to evaluate the efficiency of hybrid process for the mitigation of membrane fouling and for the optimization of pollutants removal. The experiments were performed in a continuous mode with a single pass of feed water through the membranes and without recirculation of the final retentate to the second MF/UF module. Each experimental run has been performed for the duration of 2 h, which was enough to obtain a steady membrane operation. The water to be treated was initially passing from the first module, where inlet ozone concentration was adjusted, according to the selected optimum operating conditions and subsequently, it was pumped to the second microfiltration/ultrafiltration module. All experiments were performed under stable Trans-Membrane Pressure (TMP), resulting to decrease of permeate flow rate, due to progressive membrane fouling. Microfiltration or ultrafiltration Al2O3 membranes were installed in the second membrane module. Various different liquid flow rates and TMPs were examined (1.5 L/h, 0.02–0.03 MPa TMP for microfiltration and 0.6 L/h, 0.4 MPa for ultrafiltration). Membrane fouling and pollutants removal were continuously monitored during the operation period. The flow rate was recorded every 5 min during each experiment, by using a balance and a timer. Most experiments were performed 2–3 times and the average values are shown in figures. Error values were around 1–2% and are not presented in the respective figures. After each experimental run, the filtration membranes were cleaned by soaking it in dilutions of NaOCl (50 mg/L) and C6H8O7 (100 mg/L) until the deionized water flux though the membranes was restored to the initial values. Experimental set-up (measuring devises, modules, tubing, etc.) was also repeatedly washed and cleaned with large amounts of deionized water.
3. Results and discussion Experiments performed in the present work had three major subobjectives, aiming to reach the final target, which was the development of a process combining bubbles-less ozonation and membrane filtration. Primary objective was the hydrophobic modification of aalumina membranes and the selection of the most appropriate membrane (showing minimum resistant to ozone transfer) to be used
as ozone contactor. Second, the selection of the optimum operating conditions (mainly TMP and ozone dosage) for the ozonation process and finally, the evaluation of the overall process and its comparison with single membrane filtration (especially regarding fouling mitigation, pollutants removal). 3.1. Characterization of Hydrophobized alumina membranes. Fig. 2 presents snapshots of droplets deposited on a planar membrane surface before and after modification with trichloromethylsilane. The unmodified membrane (Fig. 2a) is hydrophilic and the water droplet penetrates readily in the porous structure. On the contrary, the “triClMS” membrane is hydrophobic (Fig. 2b) and liquid penetration was not observed over time. The contact angles, penetration times and bubble point pressure (calculated by the Young–Laplace equation) for the membranes before and after modification are shown in Table 1 (Fig. 2). The modification with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was found to change only slightly the wettability of membrane surface. The modification with methyldiethoxysilane provided locally variable surface characteristics. Furthermore, the prior wetting of “MDS” modified surface induced the subsequent rapid penetration of a droplet into the porous structure of the membrane. The modification with octadecyltrimethoxysilane resulted in even lower wettability still in the hydrophilic region. In this case, it was not possible to observe droplet penetration into the membrane; possibly due to the low penetration rate, which was comparable to the evaporation rate of the droplet. Finally, the modification with trichloromethylsilane transform the membrane surface to hydrophobic with an apparent contact angle close to this of superhydrophobic surfaces (i.e. higher than 1401). No penetration of the droplet was observed in this case. Contact angle values were measured also for modified tubular membranes prepared with the solutions showed the highest hydrophobicity in the case of planar membranes (Fig. 3). The values of contact angles for tubular membranes modified with “triClMS” and “MDS” were 1431 and 1081 respectively close to the values measured for planar membranes (Table 1), indicating that the morphology of membrane had no significant impact on its hydrophobic modification (Fig. 3). Bubble-point calculations further confirmed the benefits of using hydrophobic membranes for the application of bubble-less ozonation. TMP between the gas and water phases was not
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Fig. 3. Image of the water droplet on the surface of tubular “triClMS” modified membrane. The average contact angle over the membrane was 1331.
Fig. 2. Image of a droplet of ultrapure water deposited on planar membrane surface (a) before and (b) after modification with trichloromethylsilane [40].
Table 1 Apparent macroscopic contact angles, penetration times and bubble point pressure measured before and after modification of planar ceramic membranes. Modification Contact angle [deg]
None DPPC MDS ODtriMS triClMS
Penetration time [s]
39.0 7 2.2 2.0 44.2 7 0.3 2.8 76.0 7 0.6–32.0 7 0.2 (locally variable) 63–5 75.3 7 0.6 – 143.0 7 1.7 –
Bubble point pressure [kPa] 2271 2097 698 728 o0
Abbreviations: DDPC: 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine; MDS: methyldiethoxysilane; ODtriMS: octadecyltrimethoxysilane; triClMS: trichloromethylsilane.
actually needed for ozone transfer. The main driving force is the ozone concentration difference between the two phases. A low TMP in the range of some millibars is may required in this process solely to overcome the friction during the flow of ozone in piping. “triClMS” membrane showed the highest hydrophobicity and thus to lowest resistant to ozone transfer and it was selected to be used as membrane ozone contactor at the first module of experimental unit. Furthermore, long-term tests of membrane contact with high concentrations of ozone showed that “triClMS” is chemically durable and the hydrophobic character of the membrane was not influenced by the contact with ozone (Table 1).
and mineralization. High ozone dosages results to the production of saturated by-products, such as carboxylic acids, showing low reaction rates with ozone [33]. This assumption was also confirmed by the corresponding ozone consumption percentage, as a function of mg O3/ mg TOC ratio, presented in Fig. 5. Ozone consumption was higher than 90% for O3/mg TOC ratios lower than 3.5, while for higher ozone inlet concentrations the consumption of ozone decreased rapidly. The ozone consumption was correlated with the UV254 reduction and thus, with the increase of aliphatic carbon content (Figs. 4 and 5). The optimum mg O3/mg TOC ratios were selected, based on the results of single ozonation experiments, aiming to achieve maximum ozone utilization (highest possible removal of UV254 and TOC and at the same time high ozone consumption). For the experiments with the hybrid unit and when ozonation was followed by microfiltration, a ratio of 2.1 was selected, while when it followed by ultrafiltration, this ratio was increased to 3.5. Different ratios were selected for the cases where ozonation was followed by microfiltration or ultrafiltration, because the preliminary experiments had shown that when ozonation of contaminated water was proceeded to a large extend, then the microfiltration membrane had no actual effect on the quality of permeate. The extended ozonation of humic acids reduce the size of produced fragments, which can pass through the membrane pores [34]. Similar ozonation experiments were conducted for groundwater contaminated with 44 μg/L arsenic. Trivalent arsenic oxidation proceeds fast, when ozone would be applied with a constant rate higher than 7 M s 1 [35]. Kim et al. presented that the half time of As(III) in two different groundwater's was around 4 min [36]. 2.4 mg O3/mg TOC ratio was selected for the experiments with this groundwater in order to ensure the required quantity of ozone, as well as enough contact time to achieve almost complete (495%) As(III) oxidation.
3.2. Ozonation experiments for optimum ozone dosage selection
3.3. Membrane filtration with simultaneous use of ozonation for simulated contaminated water
The results of UV254 absorbance and TOC for the simulated surface water samples after ozonation as a function of the applied mg O3/mg TOC ratio are shown in Fig. 4. A sharp decrease of absorbance at 254 nm was observed with increased ozone inlet concentrations, while almost complete UV254 reduction was achieved with mg O3/ mg TOC ratios higher than 5. Electrophile ozone reacts very fast with electron rich groups and decomposes carbon double bonds and aromatic rings, which are present in humic acid molecules and absorb at 254 nm. In contrary, minor TOC reduction was observed even for high mg O3/mg TOC ratios. A maximum reduction of only 23% was achieved for 8.3 mg O3/mg TOC ratio, indicating that although ozonation results to the almost immediately destruction of unsaturated bonds, the resulting fragments are resistant to further oxidation
Two different membranes with 100 10 9 m (microfiltration) and 3–5 10 9 m (ultrafiltration) pore diameters respectively were used as the second (in series) ceramic membrane module for the experiments with the hybrid process, as aforementioned in the experimental section. The results of UV254 absorbance reduction as a function of membrane operation time for the single membrane filtration experiments and for the hybrid process are shown in Fig. 6. The application of single membrane microfiltration was resulted in 84% UV254 reduction, while the hybrid process of microfiltration coupled with ozonation (previously applied) resulted in a slightly decline of reduction rate (77%). The decreased efficiency of the combined process, when compared to single microfiltration could be attributed to the impact of ozonation on
TOC reduction (%)
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45
50 45 40 35 30 25 20 15 10 5 0 0
1
2
3
4
5
6
7
8
9
10
UV254 reduction (%)
mg O3/mg TOC ratio 100 90 80 70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
mg O3/mg TOC ratio Fig. 4. Effect of mg O3/mg TOC ratio on UV254 absorbance and TOC% reduction during the treatment of contaminated water by the application of single ozonation.
100 95 90
90
UV254 reduction (%)
Ozone Consumption (%)
100
80
70
60
85 80 75 70 65
Microfiltration Microfiltrtion with Ozonation Ultrafiltration Ultrafiltration with Ozonation
60 55
50 1
2
3
4
5
6
7
8
mg O3/mg TOC ratio
50 0
60
120
time (min)
Fig. 5. O3 consumption under various inlet ozone concentrations during the treatment of contaminated water by the application of single ozonation.
Fig. 6. UV254 absorbance reduction during the application of single membrane filtration, as well as during the treatment by the hybrid process.
humic acid molecules. The application of ozonation can change the molecular size distribution of the humic acids, by the dissociation of higher molecular weight substances and the formation of several compounds of lower molecular weights that could more easily pass through the membrane, thus resulting to an effluent with relatively higher UV254 absorbance. A similar trend was not observed, when ultrafiltration membrane placed in the second membrane module. Both single ultrafiltration and the hybrid process resulted in higher UV254 reduction rates (around 95%). The smaller pores of ultrafiltration membrane were able to retain more effectively the fragments produced during the ozonation of humic acids (Fig. 6). The measurements of Total Organic Carbon (TOC) permeate content revealed that the hybrid process resulted in effluents with higher TOC values, when compared to the corresponding effluents produced by the application of single filtration (i.e. without ozonation) (Fig. 7). The TOC reduction rate was decreased from 76% to 65%, when microfiltration membrane was used in the second module and from 93% to 82% with the application of ultrafiltration membrane. The production of fragments with lower molecular weight during ozonation affects also the TOC content of permeates. An alternative explanation of the reduced permeate quality is the mitigation of membrane fouling. Membrane fouling by humic acids takes place mainly by pore constriction and formation of semi-permeable cake on the membrane surface and it seems that increased fouling can lead to improved humic acids retention [37]. Therefore, it can be expected
that the mitigation of fouling may increase the TOC content of the permeate. However, this assumption was not adopted in this case, as during the experiments of both single filtration or by the hybrid process the quality of permeate was not substantially influenced by the membrane operation time, as it would be expected in the previous case (Fig. 7). The variation of the other parameters during the experiments was as follows (results are not shown): pH and conductivity remained practically constant, while the turbidity and color were fully removed (100%) throughout these membranes operation. Fig. 8 depicts the changes in the flow of permeate during the single membrane filtration experiments, as well as with the hybrid process. As shown, the fouling of membrane associated to the decrease of the permeate flux was efficiently reduced by ozonation. In case of microfiltration without ozonation after 2 h of operating time, the permeate flux decreased by almost 50%, while for the hybrid experiments the corresponding reduction was reduced to 25%. The benefits of hybrid unit against the single filtration were even more obvious, when the ultrafiltration membrane was used. During the single ultrafiltration experiments the permeate flux was decreased by almost 60% after 2 h of operation, while with the hybrid process the permeate flux decreased only slightly at the beginning of filtration operation and then remained almost constant. Fouling Index calculation (as the ratio of deionized water flux through clean membrane to the corresponding flux though the fouled membrane after operation) presented in
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100 95 90 85 80 75 70 65 60 55 50 45 40 35 30
0,6 Fouling Index (FI)
TOC reduction (%)
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Microfiltration Microfiltration with Ozonation Ultrafiltration Ultrafiltration with Ozonation
0
0,3
0,0 MF+O3
MF
100
50
Fig. 7. TOC reduction during the application of single membrane filtration and during the treatment by the hybrid process.
UF+O3
Fig. 9. Fouling Indexes calculated for single membrane filtration or the hybrid process.
Table 2 Arsenic removal as a function of time during single ultrafiltration and after the application of hybrid process.
100 90 80
Operation time [min]
Ultrafiltration only [% As removal]
Ozonation and ultrafiltration [% As removal]
30 60 90 120
18.3 19.1 19.5 19.4
79.3 80.2 80.9 81.2
70 % Initial Flux (%)
UF Experiments
time (min)
60 50 40 30 20 Microfiltration
10
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prevents the absorption of them to the ceramic membrane surface [38] (Figs. 8 and 9).
0 0
100
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3.4. Treatment of groundwater contaminated with arsenic
time (min)
100 90 80
% Initial Flux (%)
70 60 50 40 30 20 Ultrafiltration with Ozonation
10
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20
40
60
80
100
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Fig. 8. Permeate flux reduction during single membrane filtration and during treatment by the hybrid process.
Fig. 9 confirmed the advantages of the hybrid process for fouling mitigation; low values of FI (around 0.35) calculated for single membrane operation experiments, while values close to 0.75 obtained with the concurrent use of ozonation, indicating increased permeates flux when ozonation applied. The mitigation of membrane fouling with the combination of ozonation and membrane filtration is also connected with the changes in the nature of humic acids during ozonation. The increase of humic acids hydrophobicity due to the transformation of unsaturated bounds and aromatic rings to hydrophilic by-products i.e. carboxylic acids (indicated by SUVA values – results are not shown)
The hybrid unit with the ultrafiltration membrane as the second ceramic membrane module was also examined for the removal of arsenic from groundwater. The results of arsenic removal for the single membrane ultrafiltration experiments, as well as for the hybrid process are shown in Table 2. The removal of arsenic was increased and reached about 80% with the simultaneous use of ozonation and ultrafiltration. The idea of combining ozonation and ultrafiltration for arsenic removal is based on the oxidation of As(III) to As(V) which is then much easier to be removed by membrane filtration, due to the different speciations, As(V) presents in water solutions as anions (e.g. H2AsO4 ), while As (III) presents in water mainly as H3AsO3 (non-dissociated). The negative charged surface of γ-alumina membranes at the experimental conditions tested (pH 7) retains As(V), due to electrostatic repulsion [39]. The 20% removal observed with the single ultrafiltration experiments is attributed to percentage of As(V), which is also present in this groundwater (data not shown) (Table 2). Turbidity, color, UV254 and TOC were practically fully removed (100%) throughout the treatment of contaminated groundwater with the hybrid process (results are not shown).
4. Conclusions In this research, the hydrophobic modification of a-Al2O3 membranes by four different polymeric solutions was examined and it was found to increase successfully the measured water contact angles from less than 391 (for the original-unmodified membrane) to maximum 1331–1431. “triClMS” modified membrane showed the relevant highest hydrophobicity (lower resistant to ozone transfer) and it was used as membrane contactor for the development of hybrid process, combining bubble-less ozonation
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and membrane filtration. Almost stoichiometric ozone consumption was achieved with the use of this modified ceramic membrane contactor and for mg O3/mg TOC ratios lower than 3.5. The benefit of using appropriate ceramic membrane contactors for ozonation is the increased energy efficiency of the process, by avoiding the requirement for destroying the excess (non-reacted) ozone. The combination of ozonation with membrane filtration was found to decrease the degree of fouling (around 25%). The removal of humic acid (as indicated by UV254 measurements) and of kaolin particles (as indicated by turbidity measurements) was nearly complete by the application of the hybrid process. However, there was a slight increase of TOC in permeates, possibly due to the decrease of molecular size distribution of organic molecules. Finally, the hybrid process was found to improve the removal of As from around 20% (single ultrailtration experiments) up to 80% resulting to concentrations lower than the legislation guidelines for drinking water (10 μg/L). Acknowledgments The financial support through co- Financed European Union and the Greek State programme EPAN-II/ESPA: 'SYNERGASIA' Project NanoMemWater, (G.R. No. 09SYN-42-440) is gratefully appreciated. The authors wish to thank Dr. Kenny Wyns, Flemish Institute for Technological Research (VITO), Belgium and Prof. Eleni Kalogianni, Alexander Technological Education Institute of Thessaloniki, Greece for contact angle measurements.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.05. 036. References [1] R.P. Schwarzenbach, The challenge of micropollutants in aquatic systems, Science 313 (2006) 1072–1077. [2] M. Köck-Schulmeyer, M. Villagrasa, M. López de Alda, R. Céspedes-Sánchez, F. Ventura, D. Barceló, Occurrence and behavior of pesticides in wastewater treatment plants and their environmental impact, Sci. Total Environ. 458–460 (2013) 466–476. [3] C.I. Kosma, D.A. Lambropoulou, T.A. Albanis, Investigation of PPCPs in wastewater treatment plants in Greece: occurrence, removal and environmental risk assessment, Sci. Total Environ. 466–467 (2014) 421–438. [4] E. Akhondi, F. Wicaksana, A.G. Fane, Evaluation of fouling deposition, fouling reversibility and energy consumption of submerged hollow fiber membrane systems with periodic backwash, J. Membr. Sci. 452 (2014) 319–331. [5] A. Al-Amoudi, R.W. Lovitt, Fouling strategies and the cleaning system of NF membranes and factors affecting cleaning efficiency, J. Membr. Sci. 303 (2007) 4–28. [6] G.S. Ajmani, D. Goodwin, K. Marsh, D.H. Fairbrother, K.J. Schwab, J.G. Jacangelo, H. Huang, Modification of low pressure membranes with carbon nanotube layers for fouling control, Water Res. 46 (2012) 5645–5654. [7] A. Maartens, P. Swart, E.P. Jacobs, Feed-water pretreatment: methods to reduce membrane fouling by natural organic matter, J. Membr. Sci. 163 (1999) 51–62. [8] E.N. Peleka, P. Mavros, A.I. Zouboulis, K.A. Matis, A hybrid flotationmicrofiltration cell for effluent treatment, Desalination 248 (2009) 881–890. [9] K. Konieczny, M. Bodzek, M. Rajca, A coagulation-MF system for water treatment using ceramic membranes, in: Proceedings of the the Second Membrane Science and Technology Conference of Visegrad Countries (PERMEA 2005), 2006, 198, pp. 92–101. [10] C. Campos, B. Mariñas, V. Snoeyink, I. Baudin, J. Lainé, PAC-membrane filtration process I: model development, J. Environ. Eng. 126 (2000) 97–103. [11] K. Szymanska, A.I. Zouboulis, D. Zamboulis, Hybrid ozonation–microfiltration system for the treatment of surface water using ceramic membrane, J. Membr. Sci. 468 (2014) 163–171.
47
[12] C. Gottschalk, J.A. Libra, A. Saupe, Ozonation of Water and Waste Water: A Practical Guide to Understanding Ozone and its Applications, Wiley-VCH,, Verlag, Weinheim, Germany, 2009. [13] B. Schlichter, V. Mavrov, H. Chmiel, Study of a hybrid process combining ozonation and microfiltration/ultrafiltration for drinking water production from surface water, Desalination 168 (2004) 307–317. [14] M. Moslemi, S.H. Davies, S.J. Masten, Bromate formation in a hybrid ozonationceramic membrane filtration system, Water Res. 45 (2011) 5529–5534. [15] F.J. Benitez, J.L. Acero, F.J. Real, C. Garcia, Combination of chemical oxidationmembrane filtration processes for the elimination of phenyl-ureas in water matrices, J. Chem. Technol. Biotechnol. 84 (2009) 1883–1893. [16] B.S. Karnik, S.H. Davies, M.J. Baumann, S.J. Masten, The effects of combined ozonation and filtration on disinfection by-product formation, Water Res. 39 (2005) 2839–2850. [17] P. Janknecht, P.A. Wilderer, C. Picard, A. Larbot, J. Sarrazin, Bubble-free ozone contacting with ceramic membranes for wet oxidative treatment, Chem. Eng. Technol. 23 (2000) 674–677. [18] A.J. Burggraaf, L. Cot, Fundamentals of inorganic membrane science and technology, in: A.J. Burggraaf, L. Cot (Eds.), Membr. Sci. Technol., Elsevier, Amsterdam, 1996, p. iii. [19] P. Janknecht, P.A. Wilderer, C. Picard, A. Larbot, Ozone-water contacting by ceramic membranes, Sep. Purif. Technol. 25 (2001) 341–346. [20] J. Phattaranawik, T. Leiknes, W. Pronk, Mass transfer studies in flat-sheet membrane contactor with ozonation, J. Membr. Sci. 247 (2005) 153–167. [21] M. Kukuzaki, K. Fujimoto, S. Kai, K. Ohe, T. Oshima, Y. Baba, Ozone mass transfer in an ozone-water contacting process with Shirasu porous glass (SPG) membranes: a comparative study of hydrophilic and hydrophobic membranes, Sep. Purif. Technol. 72 (2010) 347–356. [22] N.A. Ahmad, C.P. Leo, A.L. Ahmad, W.K.W. Ramli, Membranes with great hydrophobicity: a review on preparation and characterization, Sep. Purif. Rev. 44 (2014) 109–134. [23] C. Picard, A. Larbot, E. Tronel-Peyroz, R. Berjoan, Characterisation of hydrophilic ceramic membranes modified by fluoroalkylsilanes into hydrophobic membranes, Solid State Sci. 6 (2004) 605–612. [24] Z.D. Hendren, J. Brant, M.R. Wiesner, Surface modification of nanostructured ceramic membranes for direct contact membrane distillation, J. Membr. Sci. 331 (2009) 1–10. [25] L. Gazagnes, S. Cerneaux, M. Persin, E. Prouzet, A. Larbot, Desalination of sodium chloride solutions and seawater with hydrophobic ceramic membranes, Desalination 217 (2007) 260–266. [26] S. Heng, K.L. Yeung, M. Djafer, J.-C. Schrotter, A novel membrane reactor for ozone water treatment, J. Membr. Sci. 289 (2007) 67–75. [27] A.E. Wiącek, Changes in wetting properties of alumina surface treated with DPPC in the presence of phospholipase A2 enzyme, Colloids Surf. B: Biointerfaces 87 (2011) 54–60. [28] A. Sah, H.L. Castricum, A. Bliek, D.H.A. Blank, J.E. ten Elshof, Hydrophobic modification of γ-alumina membranes with organochlorosilanes, J. Membr. Sci. 243 (2004) 125–132. [29] J.E. Atwater, J.R. Akse, Oxygen permeation through functionalized hydrophobic tubular ceramic membranes, J. Membr. Sci. 301 (2007) 76–84. [30] D.S. Wavhal, E.R. Fisher, Membrane Surface modification by plasma-induced polymerization of acrylamide for improved surface properties and reduced protein fouling, Langmuir 19 (2002) 79–85. [31] L. Pawlowski US$120 (Hardcover), in: Arnold E. Greenberd, Lenore S. Clesceri, Andrew D. Eaton (Eds.), Standard methods for the examination of water and wastewater, 18th ed.,Water Environment Federation, Alexandria, USA, ISBN 087553-207-11992, p. 1025, Science of The Total Environment, 142 (1994) 227-228. [32] A.E. Pagana, S.D. Sklari, E.S. Kikkinides, V.T. Zaspalis, Microporous ceramic membrane technology for the removal of arsenic and chromium ions from contaminated water, Microporous and Mesoporous Mater. 110 (2008) 150–156. [33] C. Sonntag, U. von Gunten, Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications, IWA Publishing, 2012. [34] J. Peuravuori, K. Pihlaja, Molecular size distribution and spectroscopic properties of aquatic humic substances, Anal. Chim. Acta 337 (1997) 133–149. [35] U. von Gunten, Ozonation of drinking water: Part I. Oxidation kinetics and product formation, Water Res. 37 (2003) 1443–1467. [36] M.-J. Kim, J. Nriagu, Oxidation of arsenite in groundwater using ozone and oxygen, Sci. Total Environ. 247 (2000) 71–79. [37] D. Jermann, W. Pronk, S. Meylan, M. Boller, Interplay of different NOM fouling mechanisms during ultrafiltration for drinking water production, Water Res. 41 (2007) 1713–1722. [38] S. Van Geluwe, L. Braeken, B. Van der Bruggen, Ozone oxidation for the alleviation of membrane fouling by natural organic matter: a review, Water Res. 45 (2011) 3551–3570. [39] H. Park, H. Choi, As(III) removal by hybrid reactive membrane process combined with ozonation, Water Res. 45 (2011) 1933–1940. [40] S. Sklari, A. Pagana, L. Nalbandian, V. Zaspalis, Ceramic membrane materials and process for the removal of as(iii)/as(v) ions from water, J. Water Process Eng. 5 (2015) 42–47.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Development of bubble-less ozonation and membrane filtration process for the treatment of contaminated water S.K. Stylianou a, S.D. Sklari b, D. Zamboulis a, V.T. Zaspalis b,c, A.I. Zouboulis a,n a
Section of Chemical Technology, School of Chemistry, Aristotle University, GR-54124 Thessaloniki, Greece Chemical Process and Energy Resources Institute, Center for Research and Technology Hellas, GR-57001 Thessaloniki, Greece c Department of Chemical Engineering, Aristotle University, GR-54124 Thessaloniki, Greece b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 18 May 2015 Accepted 20 May 2015 Available online 29 May 2015
The aim of this research was the evaluation of performance efficiency of a continuous treatment process, integrating bubbles-less ozonation and membrane filtration for water purification. A new bench-scale experimental apparatus was designed, comprising of two ceramic membrane modules, connected in series. In the first module, bubble-free ozone contacting takes place via a modified α-Al2O3 membrane. Chemical modification of a-Al2O3 membranes was performed with four different modifying solutions in order to change their hydrophilic behavior and thus, to reduce membrane resistance to ozone transfer. The hydrophobic character of initial and modified membranes was examined by contact angle measurements. The results showed that the chemical modification of membranes decreased the hydrophilic character of the original membranes and in certain cases transform them to hydrophobic. The values of measured contact angles were between 761 and 1431. In the second treatment module (in series), a microfiltration or ultrafiltration ceramic membrane was placed in order to separate suspended particles, macromolecules and/or arsenic. The hybrid process was evaluated for the treatment of aqueous solutions, contaminated by humic acid, kaolin (clay) and/or arsenic. The results indicated that the combination of ozone with membrane filtration limits fouling (around 25%) and improves the removal of arsenic (up to 80% removal), although it had no effect on TOC reduction. & 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrophobized α-Al2O3 membranes Ozone membrane contactor Hybrid membrane processes Membrane fouling Arsenic
1. Introduction The degradation of water quality is a matter of major concern worldwide. In recent years, the construction of central water/wastewater treatment plants has provided a solution to conventional water pollution problems; however, there are several contamination cases, i. e. occurrence of heavy metals, refractory organic pollutants, etc., requiring specific/supplementary treatment, before the treated water can be used or reused [1–3]. Membrane filtration systems, which represent a relatively new and promising technology in this field, could deliver flexible and efficient solutions for smaller scale in-situ treatment, especially for slightly contaminated water. The major drawback for the wide scale implementation of membrane processes, regarding water treatment, is membrane fouling, which results to the deterioration of membrane operation, the need for fouling control and consequently, to the increase of water treatment cost [4]. Several methods have been proposed in order to address and handle membrane fouling, including physical and chemical cleaning [5], membrane surface modification [6], appropriate
n
Corresponding author. Tel./fax: þ 30 2310997794. E-mail address: [email protected] (A.I. Zouboulis).
http://dx.doi.org/10.1016/j.memsci.2015.05.036 0376-7388/& 2015 Elsevier B.V. All rights reserved.
feed water pretreatment [7], etc. An alternative approach for the mitigation of membrane fouling, as well as for the improvement of permeate quality is the implementation of hybrid membrane processes, where membrane filtration is combined with another (water) treatment method in a concurrent mode. As a result, several studies have presented the combination of membrane filtration with other water treatment processes, such as flotation [8], coagulation [9], adsorption [10], oxidation [11] etc. Ozonation was successfully applied in several applications of water/wastewater treatment field, such as in disinfection, in oxidation of refractory organic compounds, of color, of odor and taste removal etc. [12]. Ozonation in a hybrid membrane process is expected to reduce the membrane fouling rate and further to improve the quality and the biodegradability of permeate. Several studies have been published on the effect of hybrid ozonation– membrane filtration processes on fouling [13], as well as on bromated formation [14], on herbicides oxidation [15], on the formation of disinfection by-products [16] etc. In most cases, ozone transfer to the water to be treated was carried out via injectors, bubble columns and gas diffusers. Common feature of all these techniques is the formation of bubbles; however in these cases, gas-liquid transport is a rate limiting process, depending upon the ozone bubbles diameter. Such a process never results in a
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complete ozone reaction, even with smaller gas bubbles; additionally, the ozone contactor suffers from foam problems, while an ozone (mostly thermal) destructor is usually connected to the device for the post-treatment (removal) of non-reacted gas. An alternative approach is the use of appropriate membrane contactors in order to optimize the contact between the gaseous and aqueous phase and to achieve almost complete (stoichiometric) ozone consumption [17]. However, in that case only chemically inert membranes can be used, such as ceramics, due to ozone's high reactivity with the most polymeric ones. Ceramic membranes offer high chemical, thermal and mechanical stability and porosity structure and sufficient flux. The main inorganic membranes available are made from metal oxides, e.g. alumina, titania, zirconia and silica. These metal oxides are characterized by hydrophilic behavior, due to the presence of hydroxyl (OH–) groups on their surfaces. Water with contact to a hydrophilic membrane can penetrate into the pores, due to capillary forces. The level of capillary pressure in hydrophilic porous media, as foreseen from theoretical approaches, is a function of pore diameter and can reach very high values, creating problems in gas transfer through it [18]. Inversely, the suitability of hydrophobic membranes, applied for the efficient contact between gaseous and aqueous phases has been confirmed by the respective mass transfer calculations [19,20]. Kukuzaki et al. calculated that the mass transfer coefficients for hydrophbized SPG membranes were four order of magnitude higher, when compared to hydrophilic ones [21]. The hydrophobicity of ceramic membranes can be achieved through chemical modification, surface morphology modification, or by the combination of them. Hydrophobized ceramic membranes are usually prepared via chemical modification and especially, by using organosilanes as surface modifying agents [22]. In this case, several organosilanes can be used, as for example, chloroalkylsilanes, fluoroalkylsilanes (FAS), polydimethylsiloxane (PDMS), triethoxy-1H,1H,2H,2H-perfluorodecyltriethoxysilane [23–25]. The aim of this study was the development of a hybrid process, combining bubble-less ozonation and membrane filtration for water treatment. It is considered as a novel approach of a hybrid process and the relevant published studies on its performance efficiency are rather scarce [26]. The use of membrane contactor for ozonation enables rapid ozone mass transfer and uniform reaction with pollutants, avoiding the need for high ozone fluxes. Commercially available tubular α-Al2O3 membranes were chemically modified by four different hydrophobic polymers and their wetting properties were examined. Hydrophobized membranes were used as membranes contactors for the transfer of ozone to the contaminated water via diffusion phenomena (i.e. without dispersion or mixing to take place). Microfiltration or ultrafiltration alumina membranes were used as a subsequent processing separation step. Humic acids were selected as common representatives of natural organic matter in waters, creating severe fouling problems to membranes, while kaolin (clay) was used as a common source of turbidity. In addition, arsenic was selected as refractory inorganic pollutant that has been identified in alarming high concentrations in several regions around the world. The study was focused on the advantages of using a hydrophobic ceramic membrane contactor for bubble-less ozone transfer and on the effects of the hybrid process to membrane fouling and to TOC, UV254 and arsenic removals.
2. Experimental 2.1. Surface modification of contacting.
α-alumina membranes for ozone
Commercially available tubular and planar alumina membranes were used for evaluating the effectiveness of the different surface modification solutions. The tubular ceramic substrate consists of
41
three α-Al2O3 layers with a pore diameter of 900 10-9 m (base support), 500 10 9 m (intermediate layer) and 100 10-9 m (microfiltration layer), respectively. This specimen has length of 340 10 3 m, inner diameter (ID) 8 10 3 m, outer diameter (OD) 14 10 3 m, and surface 8.5 10 3 m2. The α-Al2O3 planar membrane has 50 10 3 m diameter, 80 10 9 m pore diameter and 2 10 3 m thickness; were also modified only for evaluation and comparison of their hydrophobic character in relation to the respectively modified tubular membranes. Four different hydrophobic polymers were investigated in this work for modifying the surface of ceramic membranes; 1,2dipalmitoyl-sn-glycero-3-phosphocholine (C40H80NO8P, 99% purity), trichloromethylsilane (CH3SiCl3, 97% purity), methyldiethoxysilane (C5H14O2Si, 96% purity) and octadecyltrimethoxysilane (CH3(CH2)17Si(OCH3)3, 90% purity) [27–29]. The precursors were obtained from Sigma-Aldrich and their chemical structures are illustrated in Fig. S1 (Supporting information). The hydrophobic functionalization of internal surface layer of the tubular alumina membranes was achieved via a slip casting technique, by applying a contact time of 20 s between the modification solutions and the membrane surface, by using the experimental setup described in Fig. S2 (Supporting information). The solution of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was prepared in chloroform at a concentration of 50 mg/L . 200 ml of NaCl 10 3 M was added in the film that was formed after the evaporation of CHCl3 and the new solution was passed through the internal surface of tubular membranes. The tubes were dried at room temperature overnight through evaporation in order to remove the small quality of remaining solvent, from the pores of tubular alumina membranes. The modified membranes via 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine were labeled as “DPPC”. The solution of trichloromethylsilane was prepared by dissolving 2.36 mL of the polymer in 150 mL toluene; after the membranes removed from the solution, they were rinsed five times with ethanol to remove any unreacted chemicals. The modified membranes were calcinated at 200 1C for 1 h in pure N2 atmosphere at heating rate of 0.5 1C min 1. The modified membranes via trichloromethylsilane were referred to as “triClMS”. The silanization of α-Al2O3 tubular ceramic membranes was also performed by a 95% ethanol solution, containing a 2 wt% addition of methyldiethoxysilane or octadecyltrimethoxysilane. The ethanolic solution of the silanization reagent was acidified with acetic acid to a pH of 3.3 or 4.5, respectively. The tubes were then rinsed five times with ethanol and were allowed to dry at room temperature. To complete the hydrolysis of silanization reagents, the tubes were placed in an oven at 110 1C for 30 min with heating rate of 1 1C min 1. The membranes modified via methyldiethoxysilane or octadecyltrimethoxysilane were named “MDS” or “ODtriMS”, respectively. The same procedure for hydrophobic modification was used for the case of planar membranes. These membranes were completely immersed in the same solutions as the aforementioned tubular membranes for 20 s. 2.2. Contact angle measurements. Contact angle measurements in the case of tubular ceramic membranes are more complicated, than for the planar membranes. The standard procedure for this measurement is to put a drop on the top layer of membrane and record it with a special camera. For the tubular ceramic membranes the top layer is located inside the tube. Unfortunately, this procedure causes damage in the membrane structure and the membrane cannot be used for any other purpose. For that reason, only the modified planar membranes presenting the highest hydrophobicity were selected, so as to prepare the respective tubular membranes for the measurements of contact angle.
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2.2.1. Planar membranes The macroscopic apparent contact angles of ultrapure water (resistivity at 25 1C: 18 MΩ cm) on the membrane surface were determined by the sessile drop method, using a drop shape analysis instrument (CAM 200, KSV). The measurements were performed by using 4.070.2 μL droplets. Static apparent contact angles were determined for the hydrophobic surfaces, whereas for the hydrophilic surfaces the contact angle was measured at the time of contact (o500 ms). However, the contact angle for the hydrophilic membrane surface changed drastically with time and such a contact angle measurement is not accurate. For that reason the time required for the droplet to penetrate the porous medium can be considered only as a semi-quantitative indication of differences in wettability between the different membranes [30]; the measurements were repeated 3 to 10 times and the average value with the respective standard deviation is presented in the figures. 2.2.2. Tubular membranes The contact angle of tubular membranes was measured, according to the static sessile drop method. The membranes were cut into pieces of 1 10-2 m and each piece was cut in quadrants. On each piece the contact angle was measured by placing a droplet on the membrane surface and the contact angle was measured by a high resolution camera (Data Physics Contact Angle System OCA 15 plus). A software system can analyze the angle formed between the liquid/solid and the solid/vapor interface and calculate the contact angle. For the value of the contact angle the first stable image, which it was taken was considered to represent the contact angle; with this procedure the evaporation of the water droplet was excluded, as well as the capillary intrusion into the porous material. 2.3. Experimental unit A schematic representation of the bubble-less ozonation membrane filtration experimental set up is shown in Fig. 1. It consists from two ceramic membrane modules, connected in series. Appropriate plexiglas vessels were constructed in order to house the membranes and the measuring devices. In the first module, bubble-free ozone contact (and oxidation) takes place via the modified hydrophobic aAl2O3 membrane, while at the second module, microfiltration or ultrafiltration membranes were placed in order to separate suspended particles, macromolecules (of natural organic matter, NOM) and/or metal ions. Ozone–oxygen gas mixture was produced by an ozone generator (model TOGC2A, Ozonia Triogen), where pure oxygen was used as a feed gas at a pressure of 0.05 MPa. The pressure of ozone–oxygen gas mixture produced by the ozonator was monitored by a digital pressure meter (WIKA, model S-10), whereas the flow rate of ozone–oxygen gas mixture was measured and adjusted by a flow meter, equipped with a needle valve (Aalborg, model PMR-1). Ozone– oxygen gas mixture was transferred from the outer to the inner surface of the hydrophobic a-Al2O3 membrane and then to contaminated water. A peristaltic pump (Watson Marlow, model 503U) was used to drive the contaminated water within the membrane at atmospheric pressure and at various flow rates. The calculation of ozone concentration in the influent gas mixture was taken place by passing the corresponding gas stream through a 2% KI trap, following the determination of ozone in the solution, according to the Standard Methods [31]. The residual (after treatment) dissolved ozone concentration was measured by an ozone sensor (ProMinent, type OZE). Two different membranes were used in the second module, where membrane filtration was taken place, aiming to the separation of humic acids fragments, etc., ozone oxidation products, kaolin and/or arsenic ions. An (unmodified) microfiltration a-Al2O3 membrane with an average pore diameter of 100 10 9 m (around 1000 kDa MWCO)
and an ultrafiltration γ-Al2O3 membrane with an average pore diameter of 3–5 10 9 m (around 50 kDa MWCO) were used. The γ-Al2O3 membrane was prepared via dip coating technique, as described in a previous study [32]. The ozonated water was driven through the membrane by the pressure created using an inox progressive cavity pump (Syden, model BM005-4S). TransMembrane Pressures (TMPs) were measured by a digital pressure meter (WIKA, model S-10). Permeate flux was continuously monitored by a mass balance (Kern, model FCB 8K01) (Fig. 1). 2.4. Feed water solutions/analytical methods. Two different aqueous solutions were used for the evaluation of performance efficiency of the hybrid process. For all experiments the pH and temperature of the feed solutions were adjusted at 7 and 20 1C, respectively. A synthetic solution containing 25 mg/L of humic acids and 25 mg/L of clay particles was prepared by dissolving humic acid stock solution and dispersing clay particles in the tap water of Thessaloniki city in order to simulate a medium contaminated surface water. Information about the respective concentration of major parameters in tap water of Thessaloniki is given in Table S1 (Supporting information). Humic acid (sodium salt) was obtained from Aldrich Chemical Company, while clay powder was commercially obtained. Initial samples and permeates were analyzed for the determination of the following parameters: UV absorbance at 254 nm (used as an indication of unsaturated organics concentration) and UV absorbance at 436 nm (used as an indication of color) were measured by a Hitachi UV–vis spectrophotometer; Total Organic Carbon (TOC) by a TOC-VCSH Total Organic Carbon Analyzer (Shimadzu); turbidity by a Hach Ratio/XR turbidity meter. The initial turbidity of treated water was 21.5 NTU, while the Total Organic Carbon (TOC) was 8.5 mg/L. Groundwater was collected from a drilling well, located in the settlement of Arachos in the municipality of Alexadria (outside of Thessaloniki) and it was used as the other feed water. This groundwater been found to contain a relatively high concentration of arsenic, up to 44 μg/L. Total arsenic concentrations for the initial samples and for the permeates were measured by graphite atomic absorption furnace, using PerkinElmer AAnalyst 800. In this case the initial turbidity of water was 0.4 NTU, while the Total Organic Carbon (TOC) was 0.55 mg/L. 2.5. Single ozonation experiments In order to select the optimum operating conditions for the first ceramic membrane module, single ozonation experiments were conducted, without the use of the second microfiltration/ultrafiltration module. Experiments were performed with a constant ozone flow rate of 0.03 L/min, corresponding to ozone mass flow rates of 0.7 mg/min. The low ozone flow rate was selected in order to avoid the formation of bubbles, which was among the key objectives of this study, noting that in higher ozone flow rates, the creation of rather large bubbles was observed. The feed water solutions were prepared in the influent tank, subsequently the water to be treated was driven by a peristaltic pump to the inner surface of the ceramic membrane, where it was contacted with ozone. Different inlet ozone concentrations were applied, by changing the liquid phase velocity and consequently, different contact times between ozone and contaminated water can evaluated. 2.6. Membrane filtration with simultaneous use of ozonation. After the completion of single ozonation experiments and the selection of optimum operating conditions of the first membrane
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Fig. 1. Flow chart of the hybrid unit of ozonation and membrane filtration used for contaminated water treatment. (1) O2 tank, (2) Ozone generator, (3) On-off valve, (4) Needle valve, (5) Ozone flowmeter, (6) Influent tank, (7) Peristaltic pump, (8) Water flowmeter, (9) Ozonation module, (10) Manometer, (11) Neddle valve, (12) Ozone trap, (13) Ozone analyzer, (14) Progressive Cavity pump, (15) Manometer, (16) Membrane filtration module, (17) Regulator Valve, and (18) Mass Balance.
module, additional experiments with the simultaneous use of ozonation and membrane filtration were conducted. The results obtained from the hybrid unit were compared also with the preliminary single membrane filtration tests in order to evaluate the efficiency of hybrid process for the mitigation of membrane fouling and for the optimization of pollutants removal. The experiments were performed in a continuous mode with a single pass of feed water through the membranes and without recirculation of the final retentate to the second MF/UF module. Each experimental run has been performed for the duration of 2 h, which was enough to obtain a steady membrane operation. The water to be treated was initially passing from the first module, where inlet ozone concentration was adjusted, according to the selected optimum operating conditions and subsequently, it was pumped to the second microfiltration/ultrafiltration module. All experiments were performed under stable Trans-Membrane Pressure (TMP), resulting to decrease of permeate flow rate, due to progressive membrane fouling. Microfiltration or ultrafiltration Al2O3 membranes were installed in the second membrane module. Various different liquid flow rates and TMPs were examined (1.5 L/h, 0.02–0.03 MPa TMP for microfiltration and 0.6 L/h, 0.4 MPa for ultrafiltration). Membrane fouling and pollutants removal were continuously monitored during the operation period. The flow rate was recorded every 5 min during each experiment, by using a balance and a timer. Most experiments were performed 2–3 times and the average values are shown in figures. Error values were around 1–2% and are not presented in the respective figures. After each experimental run, the filtration membranes were cleaned by soaking it in dilutions of NaOCl (50 mg/L) and C6H8O7 (100 mg/L) until the deionized water flux though the membranes was restored to the initial values. Experimental set-up (measuring devises, modules, tubing, etc.) was also repeatedly washed and cleaned with large amounts of deionized water.
3. Results and discussion Experiments performed in the present work had three major subobjectives, aiming to reach the final target, which was the development of a process combining bubbles-less ozonation and membrane filtration. Primary objective was the hydrophobic modification of aalumina membranes and the selection of the most appropriate membrane (showing minimum resistant to ozone transfer) to be used
as ozone contactor. Second, the selection of the optimum operating conditions (mainly TMP and ozone dosage) for the ozonation process and finally, the evaluation of the overall process and its comparison with single membrane filtration (especially regarding fouling mitigation, pollutants removal). 3.1. Characterization of Hydrophobized alumina membranes. Fig. 2 presents snapshots of droplets deposited on a planar membrane surface before and after modification with trichloromethylsilane. The unmodified membrane (Fig. 2a) is hydrophilic and the water droplet penetrates readily in the porous structure. On the contrary, the “triClMS” membrane is hydrophobic (Fig. 2b) and liquid penetration was not observed over time. The contact angles, penetration times and bubble point pressure (calculated by the Young–Laplace equation) for the membranes before and after modification are shown in Table 1 (Fig. 2). The modification with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was found to change only slightly the wettability of membrane surface. The modification with methyldiethoxysilane provided locally variable surface characteristics. Furthermore, the prior wetting of “MDS” modified surface induced the subsequent rapid penetration of a droplet into the porous structure of the membrane. The modification with octadecyltrimethoxysilane resulted in even lower wettability still in the hydrophilic region. In this case, it was not possible to observe droplet penetration into the membrane; possibly due to the low penetration rate, which was comparable to the evaporation rate of the droplet. Finally, the modification with trichloromethylsilane transform the membrane surface to hydrophobic with an apparent contact angle close to this of superhydrophobic surfaces (i.e. higher than 1401). No penetration of the droplet was observed in this case. Contact angle values were measured also for modified tubular membranes prepared with the solutions showed the highest hydrophobicity in the case of planar membranes (Fig. 3). The values of contact angles for tubular membranes modified with “triClMS” and “MDS” were 1431 and 1081 respectively close to the values measured for planar membranes (Table 1), indicating that the morphology of membrane had no significant impact on its hydrophobic modification (Fig. 3). Bubble-point calculations further confirmed the benefits of using hydrophobic membranes for the application of bubble-less ozonation. TMP between the gas and water phases was not
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Fig. 3. Image of the water droplet on the surface of tubular “triClMS” modified membrane. The average contact angle over the membrane was 1331.
Fig. 2. Image of a droplet of ultrapure water deposited on planar membrane surface (a) before and (b) after modification with trichloromethylsilane [40].
Table 1 Apparent macroscopic contact angles, penetration times and bubble point pressure measured before and after modification of planar ceramic membranes. Modification Contact angle [deg]
None DPPC MDS ODtriMS triClMS
Penetration time [s]
39.0 7 2.2 2.0 44.2 7 0.3 2.8 76.0 7 0.6–32.0 7 0.2 (locally variable) 63–5 75.3 7 0.6 – 143.0 7 1.7 –
Bubble point pressure [kPa] 2271 2097 698 728 o0
Abbreviations: DDPC: 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine; MDS: methyldiethoxysilane; ODtriMS: octadecyltrimethoxysilane; triClMS: trichloromethylsilane.
actually needed for ozone transfer. The main driving force is the ozone concentration difference between the two phases. A low TMP in the range of some millibars is may required in this process solely to overcome the friction during the flow of ozone in piping. “triClMS” membrane showed the highest hydrophobicity and thus to lowest resistant to ozone transfer and it was selected to be used as membrane ozone contactor at the first module of experimental unit. Furthermore, long-term tests of membrane contact with high concentrations of ozone showed that “triClMS” is chemically durable and the hydrophobic character of the membrane was not influenced by the contact with ozone (Table 1).
and mineralization. High ozone dosages results to the production of saturated by-products, such as carboxylic acids, showing low reaction rates with ozone [33]. This assumption was also confirmed by the corresponding ozone consumption percentage, as a function of mg O3/ mg TOC ratio, presented in Fig. 5. Ozone consumption was higher than 90% for O3/mg TOC ratios lower than 3.5, while for higher ozone inlet concentrations the consumption of ozone decreased rapidly. The ozone consumption was correlated with the UV254 reduction and thus, with the increase of aliphatic carbon content (Figs. 4 and 5). The optimum mg O3/mg TOC ratios were selected, based on the results of single ozonation experiments, aiming to achieve maximum ozone utilization (highest possible removal of UV254 and TOC and at the same time high ozone consumption). For the experiments with the hybrid unit and when ozonation was followed by microfiltration, a ratio of 2.1 was selected, while when it followed by ultrafiltration, this ratio was increased to 3.5. Different ratios were selected for the cases where ozonation was followed by microfiltration or ultrafiltration, because the preliminary experiments had shown that when ozonation of contaminated water was proceeded to a large extend, then the microfiltration membrane had no actual effect on the quality of permeate. The extended ozonation of humic acids reduce the size of produced fragments, which can pass through the membrane pores [34]. Similar ozonation experiments were conducted for groundwater contaminated with 44 μg/L arsenic. Trivalent arsenic oxidation proceeds fast, when ozone would be applied with a constant rate higher than 7 M s 1 [35]. Kim et al. presented that the half time of As(III) in two different groundwater's was around 4 min [36]. 2.4 mg O3/mg TOC ratio was selected for the experiments with this groundwater in order to ensure the required quantity of ozone, as well as enough contact time to achieve almost complete (495%) As(III) oxidation.
3.2. Ozonation experiments for optimum ozone dosage selection
3.3. Membrane filtration with simultaneous use of ozonation for simulated contaminated water
The results of UV254 absorbance and TOC for the simulated surface water samples after ozonation as a function of the applied mg O3/mg TOC ratio are shown in Fig. 4. A sharp decrease of absorbance at 254 nm was observed with increased ozone inlet concentrations, while almost complete UV254 reduction was achieved with mg O3/ mg TOC ratios higher than 5. Electrophile ozone reacts very fast with electron rich groups and decomposes carbon double bonds and aromatic rings, which are present in humic acid molecules and absorb at 254 nm. In contrary, minor TOC reduction was observed even for high mg O3/mg TOC ratios. A maximum reduction of only 23% was achieved for 8.3 mg O3/mg TOC ratio, indicating that although ozonation results to the almost immediately destruction of unsaturated bonds, the resulting fragments are resistant to further oxidation
Two different membranes with 100 10 9 m (microfiltration) and 3–5 10 9 m (ultrafiltration) pore diameters respectively were used as the second (in series) ceramic membrane module for the experiments with the hybrid process, as aforementioned in the experimental section. The results of UV254 absorbance reduction as a function of membrane operation time for the single membrane filtration experiments and for the hybrid process are shown in Fig. 6. The application of single membrane microfiltration was resulted in 84% UV254 reduction, while the hybrid process of microfiltration coupled with ozonation (previously applied) resulted in a slightly decline of reduction rate (77%). The decreased efficiency of the combined process, when compared to single microfiltration could be attributed to the impact of ozonation on
TOC reduction (%)
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45
50 45 40 35 30 25 20 15 10 5 0 0
1
2
3
4
5
6
7
8
9
10
UV254 reduction (%)
mg O3/mg TOC ratio 100 90 80 70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
mg O3/mg TOC ratio Fig. 4. Effect of mg O3/mg TOC ratio on UV254 absorbance and TOC% reduction during the treatment of contaminated water by the application of single ozonation.
100 95 90
90
UV254 reduction (%)
Ozone Consumption (%)
100
80
70
60
85 80 75 70 65
Microfiltration Microfiltrtion with Ozonation Ultrafiltration Ultrafiltration with Ozonation
60 55
50 1
2
3
4
5
6
7
8
mg O3/mg TOC ratio
50 0
60
120
time (min)
Fig. 5. O3 consumption under various inlet ozone concentrations during the treatment of contaminated water by the application of single ozonation.
Fig. 6. UV254 absorbance reduction during the application of single membrane filtration, as well as during the treatment by the hybrid process.
humic acid molecules. The application of ozonation can change the molecular size distribution of the humic acids, by the dissociation of higher molecular weight substances and the formation of several compounds of lower molecular weights that could more easily pass through the membrane, thus resulting to an effluent with relatively higher UV254 absorbance. A similar trend was not observed, when ultrafiltration membrane placed in the second membrane module. Both single ultrafiltration and the hybrid process resulted in higher UV254 reduction rates (around 95%). The smaller pores of ultrafiltration membrane were able to retain more effectively the fragments produced during the ozonation of humic acids (Fig. 6). The measurements of Total Organic Carbon (TOC) permeate content revealed that the hybrid process resulted in effluents with higher TOC values, when compared to the corresponding effluents produced by the application of single filtration (i.e. without ozonation) (Fig. 7). The TOC reduction rate was decreased from 76% to 65%, when microfiltration membrane was used in the second module and from 93% to 82% with the application of ultrafiltration membrane. The production of fragments with lower molecular weight during ozonation affects also the TOC content of permeates. An alternative explanation of the reduced permeate quality is the mitigation of membrane fouling. Membrane fouling by humic acids takes place mainly by pore constriction and formation of semi-permeable cake on the membrane surface and it seems that increased fouling can lead to improved humic acids retention [37]. Therefore, it can be expected
that the mitigation of fouling may increase the TOC content of the permeate. However, this assumption was not adopted in this case, as during the experiments of both single filtration or by the hybrid process the quality of permeate was not substantially influenced by the membrane operation time, as it would be expected in the previous case (Fig. 7). The variation of the other parameters during the experiments was as follows (results are not shown): pH and conductivity remained practically constant, while the turbidity and color were fully removed (100%) throughout these membranes operation. Fig. 8 depicts the changes in the flow of permeate during the single membrane filtration experiments, as well as with the hybrid process. As shown, the fouling of membrane associated to the decrease of the permeate flux was efficiently reduced by ozonation. In case of microfiltration without ozonation after 2 h of operating time, the permeate flux decreased by almost 50%, while for the hybrid experiments the corresponding reduction was reduced to 25%. The benefits of hybrid unit against the single filtration were even more obvious, when the ultrafiltration membrane was used. During the single ultrafiltration experiments the permeate flux was decreased by almost 60% after 2 h of operation, while with the hybrid process the permeate flux decreased only slightly at the beginning of filtration operation and then remained almost constant. Fouling Index calculation (as the ratio of deionized water flux through clean membrane to the corresponding flux though the fouled membrane after operation) presented in
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100 95 90 85 80 75 70 65 60 55 50 45 40 35 30
0,6 Fouling Index (FI)
TOC reduction (%)
46
Microfiltration Microfiltration with Ozonation Ultrafiltration Ultrafiltration with Ozonation
0
0,3
0,0 MF+O3
MF
100
50
Fig. 7. TOC reduction during the application of single membrane filtration and during the treatment by the hybrid process.
UF+O3
Fig. 9. Fouling Indexes calculated for single membrane filtration or the hybrid process.
Table 2 Arsenic removal as a function of time during single ultrafiltration and after the application of hybrid process.
100 90 80
Operation time [min]
Ultrafiltration only [% As removal]
Ozonation and ultrafiltration [% As removal]
30 60 90 120
18.3 19.1 19.5 19.4
79.3 80.2 80.9 81.2
70 % Initial Flux (%)
UF Experiments
time (min)
60 50 40 30 20 Microfiltration
10
Microfiltration with Ozonation
prevents the absorption of them to the ceramic membrane surface [38] (Figs. 8 and 9).
0 0
100
50
3.4. Treatment of groundwater contaminated with arsenic
time (min)
100 90 80
% Initial Flux (%)
70 60 50 40 30 20 Ultrafiltration with Ozonation
10
Ultrafiltration
0 0
20
40
60
80
100
120
time (min)
Fig. 8. Permeate flux reduction during single membrane filtration and during treatment by the hybrid process.
Fig. 9 confirmed the advantages of the hybrid process for fouling mitigation; low values of FI (around 0.35) calculated for single membrane operation experiments, while values close to 0.75 obtained with the concurrent use of ozonation, indicating increased permeates flux when ozonation applied. The mitigation of membrane fouling with the combination of ozonation and membrane filtration is also connected with the changes in the nature of humic acids during ozonation. The increase of humic acids hydrophobicity due to the transformation of unsaturated bounds and aromatic rings to hydrophilic by-products i.e. carboxylic acids (indicated by SUVA values – results are not shown)
The hybrid unit with the ultrafiltration membrane as the second ceramic membrane module was also examined for the removal of arsenic from groundwater. The results of arsenic removal for the single membrane ultrafiltration experiments, as well as for the hybrid process are shown in Table 2. The removal of arsenic was increased and reached about 80% with the simultaneous use of ozonation and ultrafiltration. The idea of combining ozonation and ultrafiltration for arsenic removal is based on the oxidation of As(III) to As(V) which is then much easier to be removed by membrane filtration, due to the different speciations, As(V) presents in water solutions as anions (e.g. H2AsO4 ), while As (III) presents in water mainly as H3AsO3 (non-dissociated). The negative charged surface of γ-alumina membranes at the experimental conditions tested (pH 7) retains As(V), due to electrostatic repulsion [39]. The 20% removal observed with the single ultrafiltration experiments is attributed to percentage of As(V), which is also present in this groundwater (data not shown) (Table 2). Turbidity, color, UV254 and TOC were practically fully removed (100%) throughout the treatment of contaminated groundwater with the hybrid process (results are not shown).
4. Conclusions In this research, the hydrophobic modification of a-Al2O3 membranes by four different polymeric solutions was examined and it was found to increase successfully the measured water contact angles from less than 391 (for the original-unmodified membrane) to maximum 1331–1431. “triClMS” modified membrane showed the relevant highest hydrophobicity (lower resistant to ozone transfer) and it was used as membrane contactor for the development of hybrid process, combining bubble-less ozonation
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and membrane filtration. Almost stoichiometric ozone consumption was achieved with the use of this modified ceramic membrane contactor and for mg O3/mg TOC ratios lower than 3.5. The benefit of using appropriate ceramic membrane contactors for ozonation is the increased energy efficiency of the process, by avoiding the requirement for destroying the excess (non-reacted) ozone. The combination of ozonation with membrane filtration was found to decrease the degree of fouling (around 25%). The removal of humic acid (as indicated by UV254 measurements) and of kaolin particles (as indicated by turbidity measurements) was nearly complete by the application of the hybrid process. However, there was a slight increase of TOC in permeates, possibly due to the decrease of molecular size distribution of organic molecules. Finally, the hybrid process was found to improve the removal of As from around 20% (single ultrailtration experiments) up to 80% resulting to concentrations lower than the legislation guidelines for drinking water (10 μg/L). Acknowledgments The financial support through co- Financed European Union and the Greek State programme EPAN-II/ESPA: 'SYNERGASIA' Project NanoMemWater, (G.R. No. 09SYN-42-440) is gratefully appreciated. The authors wish to thank Dr. Kenny Wyns, Flemish Institute for Technological Research (VITO), Belgium and Prof. Eleni Kalogianni, Alexander Technological Education Institute of Thessaloniki, Greece for contact angle measurements.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.05. 036. References [1] R.P. Schwarzenbach, The challenge of micropollutants in aquatic systems, Science 313 (2006) 1072–1077. [2] M. Köck-Schulmeyer, M. Villagrasa, M. López de Alda, R. Céspedes-Sánchez, F. Ventura, D. Barceló, Occurrence and behavior of pesticides in wastewater treatment plants and their environmental impact, Sci. Total Environ. 458–460 (2013) 466–476. [3] C.I. Kosma, D.A. Lambropoulou, T.A. Albanis, Investigation of PPCPs in wastewater treatment plants in Greece: occurrence, removal and environmental risk assessment, Sci. Total Environ. 466–467 (2014) 421–438. [4] E. Akhondi, F. Wicaksana, A.G. Fane, Evaluation of fouling deposition, fouling reversibility and energy consumption of submerged hollow fiber membrane systems with periodic backwash, J. Membr. Sci. 452 (2014) 319–331. [5] A. Al-Amoudi, R.W. Lovitt, Fouling strategies and the cleaning system of NF membranes and factors affecting cleaning efficiency, J. Membr. Sci. 303 (2007) 4–28. [6] G.S. Ajmani, D. Goodwin, K. Marsh, D.H. Fairbrother, K.J. Schwab, J.G. Jacangelo, H. Huang, Modification of low pressure membranes with carbon nanotube layers for fouling control, Water Res. 46 (2012) 5645–5654. [7] A. Maartens, P. Swart, E.P. Jacobs, Feed-water pretreatment: methods to reduce membrane fouling by natural organic matter, J. Membr. Sci. 163 (1999) 51–62. [8] E.N. Peleka, P. Mavros, A.I. Zouboulis, K.A. Matis, A hybrid flotationmicrofiltration cell for effluent treatment, Desalination 248 (2009) 881–890. [9] K. Konieczny, M. Bodzek, M. Rajca, A coagulation-MF system for water treatment using ceramic membranes, in: Proceedings of the the Second Membrane Science and Technology Conference of Visegrad Countries (PERMEA 2005), 2006, 198, pp. 92–101. [10] C. Campos, B. Mariñas, V. Snoeyink, I. Baudin, J. Lainé, PAC-membrane filtration process I: model development, J. Environ. Eng. 126 (2000) 97–103. [11] K. Szymanska, A.I. Zouboulis, D. Zamboulis, Hybrid ozonation–microfiltration system for the treatment of surface water using ceramic membrane, J. Membr. Sci. 468 (2014) 163–171.
47
[12] C. Gottschalk, J.A. Libra, A. Saupe, Ozonation of Water and Waste Water: A Practical Guide to Understanding Ozone and its Applications, Wiley-VCH,, Verlag, Weinheim, Germany, 2009. [13] B. Schlichter, V. Mavrov, H. Chmiel, Study of a hybrid process combining ozonation and microfiltration/ultrafiltration for drinking water production from surface water, Desalination 168 (2004) 307–317. [14] M. Moslemi, S.H. Davies, S.J. Masten, Bromate formation in a hybrid ozonationceramic membrane filtration system, Water Res. 45 (2011) 5529–5534. [15] F.J. Benitez, J.L. Acero, F.J. Real, C. Garcia, Combination of chemical oxidationmembrane filtration processes for the elimination of phenyl-ureas in water matrices, J. Chem. Technol. Biotechnol. 84 (2009) 1883–1893. [16] B.S. Karnik, S.H. Davies, M.J. Baumann, S.J. Masten, The effects of combined ozonation and filtration on disinfection by-product formation, Water Res. 39 (2005) 2839–2850. [17] P. Janknecht, P.A. Wilderer, C. Picard, A. Larbot, J. Sarrazin, Bubble-free ozone contacting with ceramic membranes for wet oxidative treatment, Chem. Eng. Technol. 23 (2000) 674–677. [18] A.J. Burggraaf, L. Cot, Fundamentals of inorganic membrane science and technology, in: A.J. Burggraaf, L. Cot (Eds.), Membr. Sci. Technol., Elsevier, Amsterdam, 1996, p. iii. [19] P. Janknecht, P.A. Wilderer, C. Picard, A. Larbot, Ozone-water contacting by ceramic membranes, Sep. Purif. Technol. 25 (2001) 341–346. [20] J. Phattaranawik, T. Leiknes, W. Pronk, Mass transfer studies in flat-sheet membrane contactor with ozonation, J. Membr. Sci. 247 (2005) 153–167. [21] M. Kukuzaki, K. Fujimoto, S. Kai, K. Ohe, T. Oshima, Y. Baba, Ozone mass transfer in an ozone-water contacting process with Shirasu porous glass (SPG) membranes: a comparative study of hydrophilic and hydrophobic membranes, Sep. Purif. Technol. 72 (2010) 347–356. [22] N.A. Ahmad, C.P. Leo, A.L. Ahmad, W.K.W. Ramli, Membranes with great hydrophobicity: a review on preparation and characterization, Sep. Purif. Rev. 44 (2014) 109–134. [23] C. Picard, A. Larbot, E. Tronel-Peyroz, R. Berjoan, Characterisation of hydrophilic ceramic membranes modified by fluoroalkylsilanes into hydrophobic membranes, Solid State Sci. 6 (2004) 605–612. [24] Z.D. Hendren, J. Brant, M.R. Wiesner, Surface modification of nanostructured ceramic membranes for direct contact membrane distillation, J. Membr. Sci. 331 (2009) 1–10. [25] L. Gazagnes, S. Cerneaux, M. Persin, E. Prouzet, A. Larbot, Desalination of sodium chloride solutions and seawater with hydrophobic ceramic membranes, Desalination 217 (2007) 260–266. [26] S. Heng, K.L. Yeung, M. Djafer, J.-C. Schrotter, A novel membrane reactor for ozone water treatment, J. Membr. Sci. 289 (2007) 67–75. [27] A.E. Wiącek, Changes in wetting properties of alumina surface treated with DPPC in the presence of phospholipase A2 enzyme, Colloids Surf. B: Biointerfaces 87 (2011) 54–60. [28] A. Sah, H.L. Castricum, A. Bliek, D.H.A. Blank, J.E. ten Elshof, Hydrophobic modification of γ-alumina membranes with organochlorosilanes, J. Membr. Sci. 243 (2004) 125–132. [29] J.E. Atwater, J.R. Akse, Oxygen permeation through functionalized hydrophobic tubular ceramic membranes, J. Membr. Sci. 301 (2007) 76–84. [30] D.S. Wavhal, E.R. Fisher, Membrane Surface modification by plasma-induced polymerization of acrylamide for improved surface properties and reduced protein fouling, Langmuir 19 (2002) 79–85. [31] L. Pawlowski US$120 (Hardcover), in: Arnold E. Greenberd, Lenore S. Clesceri, Andrew D. Eaton (Eds.), Standard methods for the examination of water and wastewater, 18th ed.,Water Environment Federation, Alexandria, USA, ISBN 087553-207-11992, p. 1025, Science of The Total Environment, 142 (1994) 227-228. [32] A.E. Pagana, S.D. Sklari, E.S. Kikkinides, V.T. Zaspalis, Microporous ceramic membrane technology for the removal of arsenic and chromium ions from contaminated water, Microporous and Mesoporous Mater. 110 (2008) 150–156. [33] C. Sonntag, U. von Gunten, Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications, IWA Publishing, 2012. [34] J. Peuravuori, K. Pihlaja, Molecular size distribution and spectroscopic properties of aquatic humic substances, Anal. Chim. Acta 337 (1997) 133–149. [35] U. von Gunten, Ozonation of drinking water: Part I. Oxidation kinetics and product formation, Water Res. 37 (2003) 1443–1467. [36] M.-J. Kim, J. Nriagu, Oxidation of arsenite in groundwater using ozone and oxygen, Sci. Total Environ. 247 (2000) 71–79. [37] D. Jermann, W. Pronk, S. Meylan, M. Boller, Interplay of different NOM fouling mechanisms during ultrafiltration for drinking water production, Water Res. 41 (2007) 1713–1722. [38] S. Van Geluwe, L. Braeken, B. Van der Bruggen, Ozone oxidation for the alleviation of membrane fouling by natural organic matter: a review, Water Res. 45 (2011) 3551–3570. [39] H. Park, H. Choi, As(III) removal by hybrid reactive membrane process combined with ozonation, Water Res. 45 (2011) 1933–1940. [40] S. Sklari, A. Pagana, L. Nalbandian, V. Zaspalis, Ceramic membrane materials and process for the removal of as(iii)/as(v) ions from water, J. Water Process Eng. 5 (2015) 42–47.