A pilot-scale photocatalyst-membrane hybrid reactor - Water Science ...

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Abstract We developed and tested a pilot-scale photocatalyst-membrane hybrid reactor for water treatment. The performance of the pilot-scale reactor was ...
J. Ryu*, W. Choi* and K.-H. Choo** *School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea (E-mail: [email protected]; [email protected]) **Department of Environmental Engineering, Kyungpook National University, Daegu, 702-701, Korea (E-mail: [email protected]) Abstract We developed and tested a pilot-scale photocatalyst-membrane hybrid reactor for water treatment. The performance of the pilot-scale reactor was evaluated by monitoring the degradation efficiency of several organic pollutants and the membrane suction pressure at different operating conditions. The concentration of humic acids rather increased in the initial period of UV illumination and then decreased gradually, which could be ascribed to the photoinduced desorption of humic acids from the TiO2 surface. The decoloring rate of methylene blue was faster than that of rhodamine B, whereas the order of mineralization rates of the dyes was reversed. 4-chlorophenol of 100 ppb was fully degraded under UV irradiation in 2 hours, which suggests that this hybrid reactor would be more suitable in removing micropollutants in water. The reactor was operated with either continuous or intermittent suction mode. In a continuous suction mode, the formation of TiO2 cake layers on the membrane surface occurred and caused a substantial increase in suction pressure. However, no further fouling (or suction pressure build-up) took place with an intermittent suction mode with the 9-min suction and 3-min pause period. The photocatalystmembrane hybrid reactor system developed in this study could be an attractive option for controlling micropollutants in water. Keywords Hybrid reactor; membrane; microfiltration; photocatalytic degradation; refractory pollutants; TiO2

Water Science & Technology Vol 51 No 6–7 pp 491–497 Q IWA Publishing 2005

A pilot-scale photocatalyst-membrane hybrid reactor: performance and characterization

Introduction

Semiconductor photocatalysis has been extensively investigated for its environmental applications and successfully demonstrated as a promising method of remediating polluted air and water (Hoffmann et al., 1995). The pollutants that have been degraded by semiconductor photocatalysis include a wide variety of organic compounds, inorganic and metal ions, and even biological pathogens such as virus and bacteria (Ohko et al., 2001; Nakashima et al., 2002; Lee and Choi, 2002a; Cho et al., 2004). In particular, TiO2 photocatalysts have shown an excellent performance in mineralizing recalcitrant organic pollutants due to the strong oxidizing power of its valence band (VB) holes. Even though some VB holes oxidize organic pollutants through direct charge transfer, most VB holes initiate the oxidative degradation of organic compounds through generating hydroxyl radicals on TiO2 surfaces (Choi et al., 2000; Kim and Choi, 2002b; Lee and Choi, 2002b). Despite the potential advantages of photocatalytic remediation methods, the development of an effective method to separate TiO2 particles from the treated water is prerequisite for its practical application to water treatment. Although the separation step could be obviated by immobilizing photocatalysts on supports (Molinari et al., 2000), suspensiontype reactors are more attractive because they have a substantially higher catalyst surface area and lower susceptibility to surface deactivation effects. Recently, membrane filtration has been introduced as an effective technique for separating TiO2 photocatalysts from treated water in a slurry-type reactor in which both photocatalytic degradation reactions and TiO2 separation are achieved simultaneously (Sopajaree et al., 1999a,b; Lee et al., 2001). It is expected that the membrane-based hybrid photoreactor may

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eliminate the problems such as incomplete gravitational separation of TiO2 in slurry-type reactors and instability of coated photocatalysts layers in immobilized reactors. In this study, we constructed a pilot-scale photocatalyst-membrane hybrid reactor and characterized its performance in terms of the degradation efficiency of organic pollutants and the degree of membrane fouling under various operational conditions. The feasibility of the hybrid water treatment system that combines a photocatalytic reactor with a membrane was explored in a systematic way. Materials and methods Materials and chemicals

Humic acid (HA: sodium salt, Aldrich), rhodamine B (RhB: Aldrich), methylene blue (MB: Oriental chemical industry), and 4-chlorophenol (4-CP: Aldrich) were of reagent grade and used as received. HA was initially dissolved in deionized water and subsequently filtered out through a 0.45 mm filter to remove undissolved fraction of HA. The initial concentration of HA was adjusted to approximately 3.0 mg/L of TOC by adding a calculated portion of the filtered HA stock solution into a tap-water filled reactor. Titanium dioxide (Degussa P25), a mixture of 80% anatase and 20% rutile with an average surface area of 50 m2/g, was selected as a photocatalyst, and tap water was used as a reaction medium. The membrane was a 0.1 mm hollow fiber microfiltration (MF) module with an effective surface area of 8 m2 (Mitsubishi, Japan) and the light source was a blacklight blue (BLB) lamp with a maximum emission wavelength of 365 nm (Sankyo, Japan). Experimental set-up and operational conditions

Figure 1 shows the schematic diagram of the reactor system that has a submerged membrane module and an air blower that plays the multiple roles of mixing the suspension, supplying oxygen, and inhibiting the membrane fouling. The reactor operation was controlled automatically by a programmable logic controller and continuous and intermittent suction modes were conducted to compare the membrane filtration performance. During the reactor operation, the concentration of suspended TiO2 particles and the flow rate of effluent were kept at a constant level of 0.5 ^ 0.1 g/L and 2 L/min, respectively. All experiments were carried out under the conditions as listed in Table 1 if not stated otherwise.

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Figure 1 Schematic of a pilot-scale photocatalyst-membrane hybrid reactor

Table 1 Operational parameters of the photocatalyst-membrane hybrid reactor

500 L 2 L/min (Flux = 15 L/m2 hr) 15–25 8C 10–20 kPa 300 W (30 W £ 10 EA) 100 L/min @ RT

Analytical methods

The effluent water filtrate was sampled periodically during the reactor operation. Quantitative analyses of HA, MB, and RhB were performed using an UV-VIS spectrophotometer (Hach DR4000U, USA) and a TOC analyzer (TOC – V CSH/CSN, Shimadzu, Japan). The concentration of 4-CP was measured by an HPLC (Agilent 1100 series, USA) which was equipped with an Eclipse XDB-C18 (4.6 mm £ 75 mm) column and a UV detector. The eluent solution was acetonitrile and water with 0.1% phosphoric acid. The concentration of TiO2 particles in suspension was determined from a turbidity measurement after calibration. The correlation between the turbidity and TiO2 concentration was as follows: [Turbidity, NTU] = 3,645.5 £ [TiO2 concentration, g/L] 2 57.28.

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Reactor volume Effluent flow rate Temperature Suction pressure Lamp power Air flow rate

Results and discussion Photocatalytic degradation of organic pollutants during batch operations

Photocatalytic degradation efficiencies of various organic pollutants in batchwise operation were evaluated in order to test the performance of the pilot-scale photocatalystmembrane hybrid reactor. Figure 2 shows the photocatalytic degradation profiles as a function of the operation time. It is interesting to note that the concentration of HA rather increased in the initial period of UV illumination and then decreased gradually with time, which could be ascribed to the photoinduced desorption of HA from the TiO2 surface. The adsorption of HAs on oxide surfaces reportedly increased with decreasing pH (Selli et al., 1996; Gu et al., 1994) and Cho and Choi (2002) also reported that the HA adsorption on TiO2 was strongly dependent on the pH of suspensions. While approximately 90% of HA was removed after 22 h operation on the basis of the absorbance at 254 nm (UV254), the TOC removal efficiency, as summarized in Table 2, was significantly lower than the UV removal efficiency. This indicates that the aromatic

Figure 2 Time-dependent profiles of photocatalytic degradation of several aquatic pollutants. Experimental conditions were [HA]0 = 3 ppm, [RhB]0 = 5 ppm, [MB]0 = 5 ppm, [4-CP]0 = 100 ppb, and pH = 7.0 ^ 0.3. The concentration of 4-CP was determined through HPLC analysis and those of HA, RhB, and MB were monitored spectrophotometrically at 254, 554, and 664 nm, respectively

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Table 2 TOC removal as a result of photocatalytic degradation Substrate

Humic Acid Rhodamine B Methylene Blue

Removal efficiency (%)

Operation time (h)

61.4 83.5 30

22 20.5 19

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structure of HA is destructed with an accompanying apparent decrease of UV254 but the mineralization is achieved with a much slower rate. As for dyes, the decoloring rate of MB was faster than that of RhB, whereas the TOC removal rate of RhB was faster than that of MB (Figure 2 and Table 2). This observation might be explained by the fact that the photoreduction of MB to its colorless leuco-form as well as the photocatalytic oxidation contributes to the decolorization of MB (Mills and Wang, 1999). Compared to the cases of HA and dyes whose concentrations were in the ppm level, 4-CP of 100 ppb was more rapidly degraded within 2 hours of UV irradiation: pseudofirst order removal rate constant (k) of HA, RhB, MB, and 4-CP was calculated as 0.16, 0.32, 0.55, and 1.49 h21, respectively. This result demonstrates that the photocatalystmembrane hybrid reactor can be particularly successful in controlling micropollutants. Further studies on the system design and operation for enhanced photocatalytic efficiencies are needed to make this reactor more feasible for practical water treatment. Effect of different operational modes

The reactor was operated with either continuous or intermittent suction mode while the UV irradiation was continuously provided throughout the operation time. Variations of the 4-CP concentration and suction pressure were monitored and compared among different operational modes (Figure 3). Degradation efficiencies of 4-CP were slightly reduced with intermittent operations, but there was a negligible difference in removal efficiency between the two intermittent-mode runs. On the other hand, the suction pressure build-up was markedly mitigated with the longer pause period in the later stage of operation (Figure 3b). It is thought that a pause period of suction is needed to dislodge TiO2 particle layers accumulated onto the membrane surface. The operating conditions need to be further optimized to improve the removal efficiency and, at the same time, to achieve larger treatment capabilities by increasing the flux or decreasing the pause period.

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Figure 3 Time course of (a) 4-CP degradation and (b) suction pressure variation (DP = Pt 2 P0) under different operational modes. The notation “9 min/1 min” represents an intermittent operation in which 9-min suction period is followed by 1-min pause period in repeated cycles. Experimental conditions were [TiO2] = 0.5 –0.55 g/L, [4-CP]0 = 100 ppb, pH = 7.0 ^ 0.3, temperature 15 –20 8C, P0 (9 min/1 min) = 13 kPa, P0 (9 min/3 min) = 12.6 kPa, and P0 (continuous suction) = 11.6 kPa

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Figure 4 The effect of different aeration conditions on the suspended TiO2 concentration and MB degradation. Experimental conditions were [MB]0 = 5 ppm, temperature 10 –13 8C and pH = 7.0 ^ 0.3. (1) and (2) denote different aeration conditions: the aeration in (2) is more intensive than in (1)

When Degussa P25 was used as a photocatalyst in aqueous suspensions, the optimal concentration of TiO2 was usually found to be 0.5–1.0 g/L in lab-scale testings (Kim and Choi, 2002; Lee and Choi, 2002a). Unlike the small-scale reactor in which the entire TiO2 suspension is well stirred and uniform throughout the reactor volume, the distribution of suspended TiO2 particles in large-scale reactors could be inhomogeneous and depend on the aeration condition. In the pilot-scale hybrid reactor operation, the effect of the concentration of suspended TiO2 particles on its performance needs to be examined. As shown in Figure 4, the markedly reduced concentration of suspended TiO2 particles with lower aeration rate seems to have little effect on the initial decoloring rate of MB. Although a higher TiO2 concentration in suspension was maintained with more intensive aeration by the blower, the photocatalytic reaction efficiency seems to be little affected by [TiO2] as long as the suspended TiO2 concentration is maintained above , 0.3 g/L. Therefore, it is believed that a partial reduction of suspended TiO2 concentration in the later stage of the batch operation should not significantly decrease the performance of this hybrid reactor. Longtime continuous reactor operation

Variation of the suction pressure during a continuous reactor operation was monitored to estimate the membrane performance and to determine the optimal operating mode. In

Figure 5 Variation of the suction pressure (DP = Pt 2 P0) in a long-term continuous operation. Experimental conditions were [TiO2] = 0.45 –0.55 g/L, temperature 10 –13 8C, P0 = 12.8 kPa, and pH = 7.0 ^ 0.3

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Table 3 An energy cost analysis of the photocatalyst-membrane hybrid reactor constructed in this work

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Suction pump Air blower Feed pump BLB Lamp

Power demand (kW)

Energy demand (kWh/d)

Operation timea (h/d)

0.1 0.4 0.125 0.3 (30 W £ 10)

1.8 9.6 2.25 7.2

18 24 18 24

a

Calculated on the basis of an intermittent run (9-min suction and 3-min pause)

the first stage of the continuous suction operation, there was a linear increase in suction pressure with time, probably due to the formation of TiO2 cake layers on the membrane surface, whereas an intermittent operation with the time interval of 9-min suction and 1-min pause reduced the increase rate of suction pressure as shown in Figure 5. No further increase in suction pressure (leveling off at , 6 kPa) was observed with an intermittent operation mode of 9-min suction and 3-min pause. This confirms that an intermittent operation is effective in preventing membrane fouling in longtime continuous runs. Conclusions

In this study, the performance of a pilot-scale photocatalyst-membrane hybrid reactor with 500 L volume was investigated with characterizing the removal efficiency of various organic pollutants (HA, dyes, and 4-CP) and the degree of membrane fouling. 4-CP of 100 ppb was completely photodegraded within 2-h batch operation whereas HA and dyes in the ppm level needed longer reaction times for complete decomposition. MB was decolorized faster than RhB but its mineralization rate was slower than RhB. In continuous runs, no fouling of the membrane (or no suction pressure build-up) took place with an intermittent operation with the 9-min suction and 3-min pause period. On the other hand, technical and economical aspects in water treatment should be considered to employ this hybrid reactor in place of the conventional treatment process. Even though the hybrid reactor shows good performances in degrading refractory aquatic pollutants and separating photocatalysts simultaneously, the real application of this system should depend on the treatment cost. In an energy cost analysis, the present hybrid reactor turns out to be cost-effective (ca. 9.65 kWh/m3, see Table 3) compared to other conventional processes (Owen et al., 1995). Although many terms such as capital cost, membrane replacement, and maintenance should be considered to build a complete budget, it could be competitive with conventional treatment processes when it comes to controlling refractory micropollutants in water. Acknowledgements

This research was supported by the program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce, Industry, and Energy of the Korean government.

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