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Photocatalysis-membrane hybrid system for organic removal from biologically treated sewage effluent
Separation and Purification Technology 68 (2009) 145–152
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Photocatalysis-membrane hybrid system for organic removal from biologically treated sewage effluent D.P. Ho, S. Vigneswaran ∗ , H.H. Ngo Faculty of Engineering, University of Technology, Sydney (UTS), PO Box 123, Broadway, NSW 2007, Australia
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 17 February 2009 Accepted 20 April 2009 Keywords: Photocatalysis Submerged membrane Titanium dioxide—TiO2
a b s t r a c t The application of semiconductor photocatalysis in treating wastewater has attracted growing interest due to its complete mineralisation of organic matter. Furthermore, coupling of photocatalytic process with microfiltration provided considerable advantages over the conventional methods. In this study, the photocatalytic reactivity of the catalysts was assessed at different operating conditions. The results show that the dissolved organic carbon (DOC) content was halved at a concentration of 1.0 g/L of TiO2 . With the addition of flocculant FeCl3 , the oxidation process was significantly improved further by another 30%. The recovery of TiO2 upon photooxidation process was achieved by coupling the photocatalysis reactor with a low energy submerged membrane reactor. The results show superior DOC degradation of more than 80% by this hybrid system. Moreover, it was demonstrated that photosensitization with TiO2 /UV could effectively reduce membrane fouling and enhance the permeate flux of the submerged membrane reactor. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Wastewater has long been seen as a reliable recycled source for water supply as many regions are severely facing water shortages. However, wastewater consists of various colloidal particulates, pathogenic microorganisms and organic pollutants, especially natural organic matter (NOM) which is a threat to public health. In order to be recycled and reused by the community, wastewater needs to undergo several treatment processes until a safe and reusable quality can be obtained. Advanced oxidation processes, filtration and membrane technology have been widely applied in wastewater reclamation plants. Advanced oxidation processes are typically characterized by the generation of the highly reactive hydroxyl radical (• OH) that can mineralize dissolved organic pollutants. They are: (i) ozonolysis, (ii) UV/ozone, (iii) UV/H2 O2 , (iv) irradiation with electrons, (v) photocatalysis and (vi) combinations of the above methods. Among the advanced treatment technologies, photocatalysis with TiO2 has been the focus of numerous investigations in recent years. In the UV/TiO2 photocatalytic oxidation process, hydroxyl radicals (• OH) are generated when catalyst (TiO2 ) is illuminated by ultraviolet (UV) light. As a result, organic compounds are mineralized to CO2 , H2 O and inorganic constituents [1,2]. Titanium dioxide (especially the commercialized product namely Degussa P25) is the most commonly used photocatalyst
∗ Corresponding author. E-mail address: [email protected] (S. Vigneswaran). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.04.019
material due to its strong redox ability, chemical stability and availability at low cost [3]. Generally, photocatalysis with TiO2 is considered as an advanced and green technology for its non-toxic, clean and safe properties and widely applicable in air purification, water and wastewater treatment [4]. However, photocatalysis is still limited to industrial application because of several problems such as high energy consumption and the recovery of the photocatalyst [5]. The idea of integrating the photocatalysis process with microfiltration (MF) and ultrafiltration (UF) has been intensively studied due to the perceived potential benefits that might be possible through the combination of these processes. MF not only aids in the separation of suspended catalysts but also improves the effluent quality by selective separation at molecular level. In addition, semiconductor photocatalysis is able to mineralize compounds causing membrane fouling; thus enhancing the consistency of the membrane’s operation [2,6]. On the other hand, a submerged membrane system has advantageous features of low installation cost, low energy and that they are easy to maintain. The aim of this paper is to evaluate the feasibility of integrating photocatalysis with submerged membrane system for the removal of effluent organic matter from biologically treated sewage effluent (BTSE). The effects of operating parameters (e.g. titanium dioxide (TiO2 ) concentration and irradiation time) were investigated. The performance of the slurry reactor was also assessed in terms of transmembrane pressure (TMP). The efficiency of organic matter removal using the integrated system was optimized through longrun experiments.
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Table 1 Characteristics of BTSE collected from the Sydney Olympic Park’s Water Reclamation Plant. Parameters
Unit
Value
T-N T-P NH4 -N PO4 -P TOC COD Conductivity Turbidity
mg/L mg/L mg/L mg/L mg/L mg/L s/cm NTU
2.80 3.08 0.70 1.41 11–13 35–40 246 4.19
Table 3 Characteristics of the hollow fiber membrane module. Specification Material Nominal pore size Outer diameter Inner diameter Number of fiber Length of fiber Surface area Membrane packing density Membrane manufacturer
Hydrophilic polyethylene 0.1 m 0.41 mm 0.27 mm 320 (16 × 20) 12 cm 0.05 m2 9858 m2 /m3 Mitsubishi-Rayon, Tokyo, Japan
Table 2 Physico-chemical properties of P25 Degussa (provided by the manufacturer).
2.2. Experiments
Specification
P25 Degussa TiO2
Type Structure Components
Powdered Non-porous 65% anatase, 25% rutile, 0.2% SiO2 , 0.3% Al2 O3 , 0.3% HCl, and 0.01% Fe2 O3 0.3 m 21 nm 6.9 nm 3.03 (from 500 to 300 nm) with UV–vis 130 kg/m3 49.32 ± 0.18 m2 /g Degussa, Frankfurt am Main, Germany
2.2.1. Photocatalytic oxidation The photoreactor consisted of a 1.5 L annular reactor, an UV lamp (352 nm, FL10BL, Sankyo Denki, Japan), air sparging, and a magnetic stirrer (Fig. 1). The illumination intensities of the light in the reactor varied from 46.15 to 276.96 mW/cm2 , which were calculated from the capacity of the UV lamp divided by the total surface area of 216.7 cm2 . Air sparging was equipped to supply oxygen into the reactor at the rate of 10 L/min. During the operation, the temperature was kept constant at approximately 28 ◦ C by recirculating cooling water. In the first stage of this study, several parameters were examined including catalyst doses (from 0.5 to 1.5 g/L), illumination intensity (46.15–276.96 mW/cm2 ), pH values (4.0–7.8) as well as feed flow rate (20–100 mL/min) in order to optimize the operating conditions of the photoreactor. Comparisons were evaluated in terms of organic removal efficiency for a number of key experimental conditions. The first-order kinetics given by Eq. (1) was used to describe the experimental data [9]:
Average aggregate particle diameter Primary crystal size Mean pore diameter Band gap Apparent density Specific surface area (BET) Product code
2. Experimental 2.1. Materials Biologically treated sewage effluent (BTSE) was collected from the water reclamation plant of the Sydney Olympic Park. The plant was one portion of the Water Reclamation and Management Scheme (WRAMS) which was operated since 2000 and designed for a population of approximately 20,000 people. The capacity of the reclamation plant is up to 2.2 megalitres per day [7]. The total organic carbon of BTSE ranged between 11 and 13 mg/L. Other characteristics of the BTSE are presented in Table 1. Photooxidation was conducted with powdered P25 Degussa TiO2 obtained from the Degussa Company (Germany). The TiO2 particle has a 70:30 anatase/rutile ratio and has an approximately 50 m2 /g of active surface area. In the aqueous solution, TiO2 particles aggregate and form large size agglomerations ranging from 0.2 to 1.2 m for pH of 3–8, respectively [2]. The characteristics of P25 can be found elsewhere [8] (Table 2). The membrane module used in this study was a polyethylene hydrophilic membrane (Mitsubishi-Rayon, Tokyo, Japan). The detailed specifications of the membrane provided by the manufacturer are presented in Table 3.
r=−
dC = k1 C dt
(1)
where r is the rate of photo-degradation of model pollutant (mg/L h), C is the concentration at any time (mg/L) and k1 is the first-order rate constant (h−1 ). 2.2.2. Pre-treatment by flocculation Flocculation with ferric chloride (FeCl3 ) was employed as the pre-treatment process for photooxidation to enhance the removal of organic matter. Prior to the batch photocatalysis experiments, the optimum dose of coagulant was predetermined by a standard jar test method. The solutions were placed in several 1 L Pyrex beakers and lined up on the flocculator. An increasing amount of FeCl3 was added into the beakers while the samples were stirred rapidly for 1 min at 130 rpm, followed by 20 min of slow mixing at
Fig. 1. Schematic of the batch photoreactor.
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Table 4 The relationship between catalyst doses and rate constant in the oxidation process. P25 concentration (g/L)
k (min−1 )
R2
Reff (%)
0.0 g/L 0.1 g/L 0.5 g/L 1.0 g/L 1.5 g/L
0.0009 0.0010 0.0031 0.0044 0.0047
0.8419 0.9119 0.9428 0.9431 0.9592
7 15 27 40 42
Reff = organic removal efficiency (%).
3. Results and discussion 3.1. Photocatalytic kinetics
Fig. 2. Schematic diagram of the continuous-mode photocatalysis—membrane hybrid system.
30 rpm and 40 min of flocs settling. Subsequently, the samples were filtered through a 0.45 m milipore filter and dissolved organic carbon (DOC) of the wastewater was determined using a Multi N/C (Analytik Jena) analyser. 2.2.3. Catalyst recovery by submerged membrane system The photomineralized solutions collected from the photoreactor were pumped to the submerged membrane reactor after undergoing 1 h settling in the settling tank (Fig. 2). In the submerged membrane system used, the hollow fibre membrane module was submerged in a 6 L tank. A pressure gauge was mounted on the top of the membrane to measure the transmembrane pressure (TMP). At the bottom of the tank, a soaker hose air diffuser was connected to provide air bubbles for aeration. The air flow was supplied at the rate of 19.2 m3 /(h m2 ) of membrane area. The effect of photocatalysis on membrane performance was evaluated in terms of critical flux. In this experiment, critical flux was measured quantitatively by a “flux stepping” method. Simultaneously, permeate from the membrane reactor was collected for DOC analysis. In addition, the effectiveness of TiO2 separation by the membrane was determined on the basis of turbidity removal. Turbidity was measured using a HACH 2000P turbidimeter.
The effect of catalyst concentration on photooxidation process is presented in Fig. 3. It was observed that an increase in TiO2 dose from 0.5 to 1.0 g/L significantly enhanced the degradation of the organic compounds. The effluent DOC was reduced by 40% at 1.0 g/L of TiO2 in 30 min and remained steady after 3 h operation. An addition in dose beyond 1.0 g/L did not improve the removal efficiency considerably. Therefore, 1.0 g/L of TiO2 was selected as the optimum dose considering the economical aspect. The simulation of the first-order kinetics is shown in Fig. 4 while the value of the rate constant (k) and the removal efficiency are summarized in Table 4. The plot shows that the rate constant (k) increased moderately with the increase in catalyst dose. As TiO2 concentration rose to 1.0 g/L, the rate constant (k) fluctuated around 0.0045 min−1 . In order to show that the photooxidation was the major phenomenon in this study than the adsorption in the removal of organics, the catalyst adsorption was studied under dark condition without any UV light [10]. It was found that the degradation of organic compounds due to photocatalytic oxidation took place much faster than catalyst adsorption. To achieve the same amount of DOC reduction (approximately 40%) it took approximately 90 min during dark adsorption as compared to 15 min in the batch photoreactor. Hence, adsorption phenomenon of the photocatalyst was considered to have much lower effect on the photooxidation rate. The effect of light intensity was also determined. The results show a slight variation on DOC removal with the increase in light intensity within the first 30 min. During this period, higher intensity facilitated the illumination of the photocatalysts to release the radicals faster (Fig. 5). Eventually, the rate remained steady when all particles absorbed photons and produced electron–hole pairs. Considering the energy consumption, the minimum intensity of 46.61 mW/cm2 was chosen in the subsequent experiments.
Fig. 3. Effect of TiO2 dose on the removal efficiency of organic matters from BTSE (initial TOC = 12.47 mg/L; UV light intensity = 46.61 mW/cm2 ; batch photoreactor).
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Fig. 4. Effect of TiO2 dose simulated using the first-order kinetics model (k is the rate constant (min−1 )).
Fig. 5. Effect of UV light intensity on the photooxidation process (initial TOC = 12.47 mg/L; TiO2 concentration = 1.0 g/L).
In the continuous photoreactor system, the feed flow rate was adjusted from 50 mL/min to as low as 10 mL/min (which corresponds to a retention time in the photoreactor of 30–150 min) for maximizing the photodegradation of organic compounds inside the reactor. The reactor volume was 1.5 L. The results are presented in Fig. 6. The flow rate appeared to have insignificant impact on the photooxidation rate in the range corresponding to the deten-
tion time from 30 to 150 min. The minimum time required for organic degradation in the first series of experiments was approximately 30 min. This led to an efficiency of 40–45%. After that, the organic content became relatively stable. Consequently, the highest flow rate of 50 mL/min was selected as it would yield the highest productivity. This corresponds to a detention time of 30 min.
Fig. 6. Effect of flow rate (detention time) on the removal efficiency of organic matter from BTSE (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; reactor volume = 1.5 L; DT = detention time).
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Fig. 7. Variations of pH and turbidity at different doses of FeCl3 (initial DOC = 10.57 mg/L).
The obtained operating parameters were compared with previous studies on UV/TiO2 [6,11–12]. Most of the studies reported the optimal dose of catalysts ranging from 0.1 to 2.0 g/L depending on the model pollutants. For decolourisation and the treatment of turbid water, TiO2 was found as low as 0.1–0.5 g/L [6]. However, in the case of persistent organics in biologically treated wastewater, a higher amount of catalyst was required [2,12]. Furthermore, similar conditions were reported with an annular photoreactor for synthetic wastewater [11]. In their studies, the DOC removal efficiency was obtained at 60% at the flow rate of 40 mL/min [11]. The lower efficiency could be due to a number of factors including the different reactor configurations, fluid flow gap, mixing conditions as well as the complexity of the real wastewater compared to synthetic wastewater. 3.2. Effect of pre-treatment on photooxidation process It was found in the previous experiments as well as in this study that the photocatalytic oxidation as a sole process led to only a partial DOC removal (40–50%). Thus, pre-treatment by FeCl3 flocculation was employed to improve the degradation rate of photocatalysis and the effluent quality. Ferric chloride (FeCl3 ) was chosen as the flocculant due to its availability and capability to remove both colloidal and organic matters. Previous studies have shown that the presence of ferrous ions, silver ions, manganese ions, etc. can significantly improve the treatment efficiency of photocatalytic oxidation [13,14]. The optimum FeCl3 dose was predetermined by jar-test and these results are shown in Figs. 7 and 8.
Due to its mild oxidizing and acidic property, pH of the solution was changed accordingly to the quantity of FeCl3 added. A virtually linear correlation between coagulant dose and pH was demonstrated in Fig. 7. The phenomenon of pH dependence was reported by Glaze et al. [15] and Cabrera et al. [16] in the treatment of trichloroethylene (TCE) using photocatalysis. The enhancement of photooxidation at low suspension pH was attributed to the protonation of the radical anions on the TiO2 surface [17]. The turbidity of the solutions after 45 min of floc settling was also measured. It was observed that inadequate doses of FeCl3 led to insufficient floc formation which resulted in higher turbidity. Fig. 7 shows that the optimum FeCl3 dose was less than 60 mg/L (or 20 mg/L as Fe+3 ). Higher dose resulted in an increase in the turbidity of the settled solutions up to 10–12 NTU. Pre-flocculation at an optimal dose led to approximately 25–30% removal of DOC at a low turbidity of 1.3 NTU. At optimal dose of FeCl3 of 18–20 mg/L of Fe+3 , the pH ranged between 5.5 and 6.0 which is only slightly below the pH limit of discharges. After the settling of flocs, the solution was fed into the photoreactor for further degradation of organic matter. The results are presented in Fig. 8. As the dose of FeCl3 was increased from 30 to 100 mg/L, a moderate increase in DOC removal was obtained. It was found that pre-flocculation removed about 20% of colloidal particles from the BTSE. An additional 60% of DOC removal efficiency was achieved by UV/TiO2 photooxidation at a FeCl3 dose of 50 mg/L. However, an increase in further amount of FeCl3 led to only a marginal improvement in the degradation rate of organic. Therefore, the optimal FeCl3 dose was taken as 50 mg/L.
Fig. 8. Effect of flocculation as a pre-treatment to photocatalytic process (TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; initial DOC = 12.67 mg/L).
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Fig. 9. Effect of photocatalytic oxidation on critical flux (TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; membrane pore size = 0.1 m; initial TOC = 12.65 mg/L).
Fig. 10. Effect of pre-treatment on the TMP development (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L; FeCl3 dose = 50 mg/L; backwash frequency = 1 h, backwash duration = 1 min; backwash rate = 2.5 times of filtration flux; PP = photocatalysis).
3.3. Membrane fouling improvement by photooxidation Several studies have suggested that organic matter is the key component affecting the membrane fouling [18]. As a consequence, photomineralizing of complex organic compounds by UV/TiO2 prior
to membrane filtration would significantly reduce fouling, enhance the performance of the membrane and lengthen its service life. The reduction of fouling was observed when the system was operated at a critical flux as shown in Fig. 9 (both with and without photocatalysis). The filtration flux was almost doubled upon the photocatalytic
Fig. 11. DOC removal efficiency of the photocatalysis—membrane integrated system with pre-flocculation (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L, light intensity = 46.61 mW/cm2 ; feed flow-rate = 50 mL/min; settling time of TiO2 = 1 h; FeCl3 = 50 mg/L).
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Fig. 12. HPLC analysis of the biologically treated sewage effluent (BTSE) by different treatment (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; radiation duration = 3 h; filtration flux = 20 L/m2 h).
oxidation and the submerged membrane reactor was able to operate at a constant flux for 5 h with a backwash frequency of 1 h. Although critical filtration flux observed after flocculation and photocatalysis also remained at 40 L/m2 h (same flux as that with a pre-treatment of photocatalysis only), the transmembrane pressure (TMP) of the submerged module was found to be comparatively low (Fig. 10). 3.4. Performance of the integrated photocatalysis and membrane system The synergic effects of the integrated photocatalysis— membrane system with pre-flocculation were evaluated in terms of organic removal efficiency (Fig. 11). Membrane filtration alone was able to remove about 20% of dissolved organics at a low filtration flux of 20 L/m2 h. When it was coupled with photocatalytic oxidation, the organic degradation efficiency of the integrated system and the filtration flux of the membrane reactor were doubled. Membrane fouling was significantly reduced and the performance was enhanced. Pre-flocculation with 50 mg/L of FeCl3 was found to effectively facilitate the degradation of organic matter by the photocatalytic oxidation process. The TOC removal of the hybrid system increased to 80%. The molecular weight distribution of organic matter after the pre-treatment was measured using high performance liquid chromatography (HPLC) analysis. Besides the major peak at the MW of 1500–2000 Da, there emerged another apparent peak at 100 Da and a minor one at 900 Da (Fig. 12). As can be seen from Fig. 10, membrane filtration and photocatalysis were able to remove large molecules rather than small ones. However, when pre-flocculation was applied, the absorbance intensities of these peaks reduced significantly. It indicated that pre-flocculation effectively enhanced the degradation of large and small molecular weight fractions. Moreover, no by-products were detected by the HPLC. 4. Conclusions The photocatalytic oxidation of organic compounds in BTSE was studied at varying TiO2 amount and other operating conditions. Pre-treatment by flocculation and membrane filtration were also employed in the treatment system. Several conclusions made from this study are as follows:
• The photocatalytic oxidation process was significantly affected by catalyst doses and only marginally by the detention time. Higher catalyst doses were able to facilitate more radicals and faster reactions to occur. At the optimal operating conditions, photocatalysis by P25/UV irradiation yielded approximately 42% of DOC removal efficiency after 120 min. • Pre-treatment by flocculation was found to enhance the degradation of organic compounds and improve the effluent quality. The removal efficiency was increased by 40% after flocculation, thus providing a total efficiency up to 80%. However, the pH readings of treated water ranged between 5.5 and 6.5, which were lower than the limit of discharged water. Therefore, another step to adjust pH of the effluent is required that may raise in the overall cost. • The critical flux of the submerged membrane increased from 25 to 40 L/m2 h after a pre-treatment of photocatalysis. Prephotooxidation with TiO2 /UV could effectively reduce membrane fouling and enhance the critical flux of the submerged membrane reactor. In addition, the HPLC analysis demonstrated that the integration of photocatalysis and microfiltration was able to remove both large (1500–2000 Da) and small MW organics (100–200 Da). Acknowledgement This research was funded by Australian Research Council (ARC) Discovery Grant DP0666257. References [1] S. Parsons, Advanced oxidation processes in water and wastewater treatment, IWA Publishing, London, 2004. [2] H.K. Shon, S. Vigneswaran, H.H. Ngo, J.H. Kim, Chemical coupling of photocatalysis with flocculation and adsorption in the removal of organic matter, Wat. Res. 39 (2005) 2549–2558. [3] D.S. Bhatkhande, V.G. Pangarkar, A. Beenackers, Photocatalytic degradation for environmental applications—a review, Chem. Technol. Biotechnol. 77 (2001) 102–116. [4] J. Arana, J.A.H. Melian, J.M.D. Rodriguez, O.G. Diaz, A. Viera, J.P. Pena, P.M.M. Sosa, V.E. Jimenez, TiO2 -photocatalysis as a tertiary treatment of naturally treated wastewater, Catal. Today 76 (2002) 279–289. [5] R. Pelton, X. Geng, M. Brook, Photocatalytic paper from colloidal TiO2 —fact of fantasy, Adv. Colloid Interf. Sci. 127 (2006) 42–53. [6] S. Mozia, A.W. Morawski, Hybridization of photocatalysis and membrane distillation for purification of wastewater, Catal. Today 118 (2006) 181–188. [7] Sydney Olympic Park Authority (SOPA), Review of Water and Wastewater Industry Structure and Pricing in the Greater Sydney Metropolitan Area, 2005, http://www.ipart.nsw.gov.au/. [8] J. Kleine, K.V. Peinemann, C. Schuster, H.J. Warnecke, Multifunctional system for treatment of wastewaters from adhesive-producing industries: separation
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[13] V. Sarria, P. Peringer, J. Caceres, J. Blanco, S. Malato, C. Pulgarin, Solar degradation of 5-amino-6-methyl-2-benzimidazolone by TiO2 and iron (III) catalyst with H2 O2 and O2 as electron acceptors, Energy 29 (2004) 853–860. [14] W. Baran, A. Makowski, W. Wardas, The separation of catalyst after photocatalytic reactions conducted in the presence of TiO2 /FeCl3 /UV, Chemosphere 59 (2005) 853–859. [15] W.H. Glaze, J.F. Kennekem, J.L. Ferry, Chlorinated by-products from the TiO2 mediated photodegradation of trichloroethylene and tetrachloroethylene in water, Environ. Sci. Technol. 27 (1993) 177–184. [16] M.I. Cabrera, A.C. Negro, O.M. Alfano, A.E. Cassano, Photocatalytic reactions involving hydroxyl radical attack, J. Catal. 172 (1997) 380–390. [17] C. Tang, V. Chen, The photocatalytic degradation of reactive black 5 using TiO2 /UV in an annular photoreactor, Wat. Res. 38 (2004) 2775–2781. [18] I.S. Chang, S.J. Judd, Air sparging of a submerged MBR for municipal wastewater treatment, Process Biochem. 37 (2002) 915–920.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Photocatalysis-membrane hybrid system for organic removal from biologically treated sewage effluent D.P. Ho, S. Vigneswaran ∗ , H.H. Ngo Faculty of Engineering, University of Technology, Sydney (UTS), PO Box 123, Broadway, NSW 2007, Australia
a r t i c l e
i n f o
Article history: Received 30 December 2008 Received in revised form 17 February 2009 Accepted 20 April 2009 Keywords: Photocatalysis Submerged membrane Titanium dioxide—TiO2
a b s t r a c t The application of semiconductor photocatalysis in treating wastewater has attracted growing interest due to its complete mineralisation of organic matter. Furthermore, coupling of photocatalytic process with microfiltration provided considerable advantages over the conventional methods. In this study, the photocatalytic reactivity of the catalysts was assessed at different operating conditions. The results show that the dissolved organic carbon (DOC) content was halved at a concentration of 1.0 g/L of TiO2 . With the addition of flocculant FeCl3 , the oxidation process was significantly improved further by another 30%. The recovery of TiO2 upon photooxidation process was achieved by coupling the photocatalysis reactor with a low energy submerged membrane reactor. The results show superior DOC degradation of more than 80% by this hybrid system. Moreover, it was demonstrated that photosensitization with TiO2 /UV could effectively reduce membrane fouling and enhance the permeate flux of the submerged membrane reactor. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Wastewater has long been seen as a reliable recycled source for water supply as many regions are severely facing water shortages. However, wastewater consists of various colloidal particulates, pathogenic microorganisms and organic pollutants, especially natural organic matter (NOM) which is a threat to public health. In order to be recycled and reused by the community, wastewater needs to undergo several treatment processes until a safe and reusable quality can be obtained. Advanced oxidation processes, filtration and membrane technology have been widely applied in wastewater reclamation plants. Advanced oxidation processes are typically characterized by the generation of the highly reactive hydroxyl radical (• OH) that can mineralize dissolved organic pollutants. They are: (i) ozonolysis, (ii) UV/ozone, (iii) UV/H2 O2 , (iv) irradiation with electrons, (v) photocatalysis and (vi) combinations of the above methods. Among the advanced treatment technologies, photocatalysis with TiO2 has been the focus of numerous investigations in recent years. In the UV/TiO2 photocatalytic oxidation process, hydroxyl radicals (• OH) are generated when catalyst (TiO2 ) is illuminated by ultraviolet (UV) light. As a result, organic compounds are mineralized to CO2 , H2 O and inorganic constituents [1,2]. Titanium dioxide (especially the commercialized product namely Degussa P25) is the most commonly used photocatalyst
∗ Corresponding author. E-mail address: [email protected] (S. Vigneswaran). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.04.019
material due to its strong redox ability, chemical stability and availability at low cost [3]. Generally, photocatalysis with TiO2 is considered as an advanced and green technology for its non-toxic, clean and safe properties and widely applicable in air purification, water and wastewater treatment [4]. However, photocatalysis is still limited to industrial application because of several problems such as high energy consumption and the recovery of the photocatalyst [5]. The idea of integrating the photocatalysis process with microfiltration (MF) and ultrafiltration (UF) has been intensively studied due to the perceived potential benefits that might be possible through the combination of these processes. MF not only aids in the separation of suspended catalysts but also improves the effluent quality by selective separation at molecular level. In addition, semiconductor photocatalysis is able to mineralize compounds causing membrane fouling; thus enhancing the consistency of the membrane’s operation [2,6]. On the other hand, a submerged membrane system has advantageous features of low installation cost, low energy and that they are easy to maintain. The aim of this paper is to evaluate the feasibility of integrating photocatalysis with submerged membrane system for the removal of effluent organic matter from biologically treated sewage effluent (BTSE). The effects of operating parameters (e.g. titanium dioxide (TiO2 ) concentration and irradiation time) were investigated. The performance of the slurry reactor was also assessed in terms of transmembrane pressure (TMP). The efficiency of organic matter removal using the integrated system was optimized through longrun experiments.
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Table 1 Characteristics of BTSE collected from the Sydney Olympic Park’s Water Reclamation Plant. Parameters
Unit
Value
T-N T-P NH4 -N PO4 -P TOC COD Conductivity Turbidity
mg/L mg/L mg/L mg/L mg/L mg/L s/cm NTU
2.80 3.08 0.70 1.41 11–13 35–40 246 4.19
Table 3 Characteristics of the hollow fiber membrane module. Specification Material Nominal pore size Outer diameter Inner diameter Number of fiber Length of fiber Surface area Membrane packing density Membrane manufacturer
Hydrophilic polyethylene 0.1 m 0.41 mm 0.27 mm 320 (16 × 20) 12 cm 0.05 m2 9858 m2 /m3 Mitsubishi-Rayon, Tokyo, Japan
Table 2 Physico-chemical properties of P25 Degussa (provided by the manufacturer).
2.2. Experiments
Specification
P25 Degussa TiO2
Type Structure Components
Powdered Non-porous 65% anatase, 25% rutile, 0.2% SiO2 , 0.3% Al2 O3 , 0.3% HCl, and 0.01% Fe2 O3 0.3 m 21 nm 6.9 nm 3.03 (from 500 to 300 nm) with UV–vis 130 kg/m3 49.32 ± 0.18 m2 /g Degussa, Frankfurt am Main, Germany
2.2.1. Photocatalytic oxidation The photoreactor consisted of a 1.5 L annular reactor, an UV lamp (352 nm, FL10BL, Sankyo Denki, Japan), air sparging, and a magnetic stirrer (Fig. 1). The illumination intensities of the light in the reactor varied from 46.15 to 276.96 mW/cm2 , which were calculated from the capacity of the UV lamp divided by the total surface area of 216.7 cm2 . Air sparging was equipped to supply oxygen into the reactor at the rate of 10 L/min. During the operation, the temperature was kept constant at approximately 28 ◦ C by recirculating cooling water. In the first stage of this study, several parameters were examined including catalyst doses (from 0.5 to 1.5 g/L), illumination intensity (46.15–276.96 mW/cm2 ), pH values (4.0–7.8) as well as feed flow rate (20–100 mL/min) in order to optimize the operating conditions of the photoreactor. Comparisons were evaluated in terms of organic removal efficiency for a number of key experimental conditions. The first-order kinetics given by Eq. (1) was used to describe the experimental data [9]:
Average aggregate particle diameter Primary crystal size Mean pore diameter Band gap Apparent density Specific surface area (BET) Product code
2. Experimental 2.1. Materials Biologically treated sewage effluent (BTSE) was collected from the water reclamation plant of the Sydney Olympic Park. The plant was one portion of the Water Reclamation and Management Scheme (WRAMS) which was operated since 2000 and designed for a population of approximately 20,000 people. The capacity of the reclamation plant is up to 2.2 megalitres per day [7]. The total organic carbon of BTSE ranged between 11 and 13 mg/L. Other characteristics of the BTSE are presented in Table 1. Photooxidation was conducted with powdered P25 Degussa TiO2 obtained from the Degussa Company (Germany). The TiO2 particle has a 70:30 anatase/rutile ratio and has an approximately 50 m2 /g of active surface area. In the aqueous solution, TiO2 particles aggregate and form large size agglomerations ranging from 0.2 to 1.2 m for pH of 3–8, respectively [2]. The characteristics of P25 can be found elsewhere [8] (Table 2). The membrane module used in this study was a polyethylene hydrophilic membrane (Mitsubishi-Rayon, Tokyo, Japan). The detailed specifications of the membrane provided by the manufacturer are presented in Table 3.
r=−
dC = k1 C dt
(1)
where r is the rate of photo-degradation of model pollutant (mg/L h), C is the concentration at any time (mg/L) and k1 is the first-order rate constant (h−1 ). 2.2.2. Pre-treatment by flocculation Flocculation with ferric chloride (FeCl3 ) was employed as the pre-treatment process for photooxidation to enhance the removal of organic matter. Prior to the batch photocatalysis experiments, the optimum dose of coagulant was predetermined by a standard jar test method. The solutions were placed in several 1 L Pyrex beakers and lined up on the flocculator. An increasing amount of FeCl3 was added into the beakers while the samples were stirred rapidly for 1 min at 130 rpm, followed by 20 min of slow mixing at
Fig. 1. Schematic of the batch photoreactor.
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Table 4 The relationship between catalyst doses and rate constant in the oxidation process. P25 concentration (g/L)
k (min−1 )
R2
Reff (%)
0.0 g/L 0.1 g/L 0.5 g/L 1.0 g/L 1.5 g/L
0.0009 0.0010 0.0031 0.0044 0.0047
0.8419 0.9119 0.9428 0.9431 0.9592
7 15 27 40 42
Reff = organic removal efficiency (%).
3. Results and discussion 3.1. Photocatalytic kinetics
Fig. 2. Schematic diagram of the continuous-mode photocatalysis—membrane hybrid system.
30 rpm and 40 min of flocs settling. Subsequently, the samples were filtered through a 0.45 m milipore filter and dissolved organic carbon (DOC) of the wastewater was determined using a Multi N/C (Analytik Jena) analyser. 2.2.3. Catalyst recovery by submerged membrane system The photomineralized solutions collected from the photoreactor were pumped to the submerged membrane reactor after undergoing 1 h settling in the settling tank (Fig. 2). In the submerged membrane system used, the hollow fibre membrane module was submerged in a 6 L tank. A pressure gauge was mounted on the top of the membrane to measure the transmembrane pressure (TMP). At the bottom of the tank, a soaker hose air diffuser was connected to provide air bubbles for aeration. The air flow was supplied at the rate of 19.2 m3 /(h m2 ) of membrane area. The effect of photocatalysis on membrane performance was evaluated in terms of critical flux. In this experiment, critical flux was measured quantitatively by a “flux stepping” method. Simultaneously, permeate from the membrane reactor was collected for DOC analysis. In addition, the effectiveness of TiO2 separation by the membrane was determined on the basis of turbidity removal. Turbidity was measured using a HACH 2000P turbidimeter.
The effect of catalyst concentration on photooxidation process is presented in Fig. 3. It was observed that an increase in TiO2 dose from 0.5 to 1.0 g/L significantly enhanced the degradation of the organic compounds. The effluent DOC was reduced by 40% at 1.0 g/L of TiO2 in 30 min and remained steady after 3 h operation. An addition in dose beyond 1.0 g/L did not improve the removal efficiency considerably. Therefore, 1.0 g/L of TiO2 was selected as the optimum dose considering the economical aspect. The simulation of the first-order kinetics is shown in Fig. 4 while the value of the rate constant (k) and the removal efficiency are summarized in Table 4. The plot shows that the rate constant (k) increased moderately with the increase in catalyst dose. As TiO2 concentration rose to 1.0 g/L, the rate constant (k) fluctuated around 0.0045 min−1 . In order to show that the photooxidation was the major phenomenon in this study than the adsorption in the removal of organics, the catalyst adsorption was studied under dark condition without any UV light [10]. It was found that the degradation of organic compounds due to photocatalytic oxidation took place much faster than catalyst adsorption. To achieve the same amount of DOC reduction (approximately 40%) it took approximately 90 min during dark adsorption as compared to 15 min in the batch photoreactor. Hence, adsorption phenomenon of the photocatalyst was considered to have much lower effect on the photooxidation rate. The effect of light intensity was also determined. The results show a slight variation on DOC removal with the increase in light intensity within the first 30 min. During this period, higher intensity facilitated the illumination of the photocatalysts to release the radicals faster (Fig. 5). Eventually, the rate remained steady when all particles absorbed photons and produced electron–hole pairs. Considering the energy consumption, the minimum intensity of 46.61 mW/cm2 was chosen in the subsequent experiments.
Fig. 3. Effect of TiO2 dose on the removal efficiency of organic matters from BTSE (initial TOC = 12.47 mg/L; UV light intensity = 46.61 mW/cm2 ; batch photoreactor).
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Fig. 4. Effect of TiO2 dose simulated using the first-order kinetics model (k is the rate constant (min−1 )).
Fig. 5. Effect of UV light intensity on the photooxidation process (initial TOC = 12.47 mg/L; TiO2 concentration = 1.0 g/L).
In the continuous photoreactor system, the feed flow rate was adjusted from 50 mL/min to as low as 10 mL/min (which corresponds to a retention time in the photoreactor of 30–150 min) for maximizing the photodegradation of organic compounds inside the reactor. The reactor volume was 1.5 L. The results are presented in Fig. 6. The flow rate appeared to have insignificant impact on the photooxidation rate in the range corresponding to the deten-
tion time from 30 to 150 min. The minimum time required for organic degradation in the first series of experiments was approximately 30 min. This led to an efficiency of 40–45%. After that, the organic content became relatively stable. Consequently, the highest flow rate of 50 mL/min was selected as it would yield the highest productivity. This corresponds to a detention time of 30 min.
Fig. 6. Effect of flow rate (detention time) on the removal efficiency of organic matter from BTSE (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; reactor volume = 1.5 L; DT = detention time).
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Fig. 7. Variations of pH and turbidity at different doses of FeCl3 (initial DOC = 10.57 mg/L).
The obtained operating parameters were compared with previous studies on UV/TiO2 [6,11–12]. Most of the studies reported the optimal dose of catalysts ranging from 0.1 to 2.0 g/L depending on the model pollutants. For decolourisation and the treatment of turbid water, TiO2 was found as low as 0.1–0.5 g/L [6]. However, in the case of persistent organics in biologically treated wastewater, a higher amount of catalyst was required [2,12]. Furthermore, similar conditions were reported with an annular photoreactor for synthetic wastewater [11]. In their studies, the DOC removal efficiency was obtained at 60% at the flow rate of 40 mL/min [11]. The lower efficiency could be due to a number of factors including the different reactor configurations, fluid flow gap, mixing conditions as well as the complexity of the real wastewater compared to synthetic wastewater. 3.2. Effect of pre-treatment on photooxidation process It was found in the previous experiments as well as in this study that the photocatalytic oxidation as a sole process led to only a partial DOC removal (40–50%). Thus, pre-treatment by FeCl3 flocculation was employed to improve the degradation rate of photocatalysis and the effluent quality. Ferric chloride (FeCl3 ) was chosen as the flocculant due to its availability and capability to remove both colloidal and organic matters. Previous studies have shown that the presence of ferrous ions, silver ions, manganese ions, etc. can significantly improve the treatment efficiency of photocatalytic oxidation [13,14]. The optimum FeCl3 dose was predetermined by jar-test and these results are shown in Figs. 7 and 8.
Due to its mild oxidizing and acidic property, pH of the solution was changed accordingly to the quantity of FeCl3 added. A virtually linear correlation between coagulant dose and pH was demonstrated in Fig. 7. The phenomenon of pH dependence was reported by Glaze et al. [15] and Cabrera et al. [16] in the treatment of trichloroethylene (TCE) using photocatalysis. The enhancement of photooxidation at low suspension pH was attributed to the protonation of the radical anions on the TiO2 surface [17]. The turbidity of the solutions after 45 min of floc settling was also measured. It was observed that inadequate doses of FeCl3 led to insufficient floc formation which resulted in higher turbidity. Fig. 7 shows that the optimum FeCl3 dose was less than 60 mg/L (or 20 mg/L as Fe+3 ). Higher dose resulted in an increase in the turbidity of the settled solutions up to 10–12 NTU. Pre-flocculation at an optimal dose led to approximately 25–30% removal of DOC at a low turbidity of 1.3 NTU. At optimal dose of FeCl3 of 18–20 mg/L of Fe+3 , the pH ranged between 5.5 and 6.0 which is only slightly below the pH limit of discharges. After the settling of flocs, the solution was fed into the photoreactor for further degradation of organic matter. The results are presented in Fig. 8. As the dose of FeCl3 was increased from 30 to 100 mg/L, a moderate increase in DOC removal was obtained. It was found that pre-flocculation removed about 20% of colloidal particles from the BTSE. An additional 60% of DOC removal efficiency was achieved by UV/TiO2 photooxidation at a FeCl3 dose of 50 mg/L. However, an increase in further amount of FeCl3 led to only a marginal improvement in the degradation rate of organic. Therefore, the optimal FeCl3 dose was taken as 50 mg/L.
Fig. 8. Effect of flocculation as a pre-treatment to photocatalytic process (TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; initial DOC = 12.67 mg/L).
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Fig. 9. Effect of photocatalytic oxidation on critical flux (TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; membrane pore size = 0.1 m; initial TOC = 12.65 mg/L).
Fig. 10. Effect of pre-treatment on the TMP development (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L; FeCl3 dose = 50 mg/L; backwash frequency = 1 h, backwash duration = 1 min; backwash rate = 2.5 times of filtration flux; PP = photocatalysis).
3.3. Membrane fouling improvement by photooxidation Several studies have suggested that organic matter is the key component affecting the membrane fouling [18]. As a consequence, photomineralizing of complex organic compounds by UV/TiO2 prior
to membrane filtration would significantly reduce fouling, enhance the performance of the membrane and lengthen its service life. The reduction of fouling was observed when the system was operated at a critical flux as shown in Fig. 9 (both with and without photocatalysis). The filtration flux was almost doubled upon the photocatalytic
Fig. 11. DOC removal efficiency of the photocatalysis—membrane integrated system with pre-flocculation (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L, light intensity = 46.61 mW/cm2 ; feed flow-rate = 50 mL/min; settling time of TiO2 = 1 h; FeCl3 = 50 mg/L).
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Fig. 12. HPLC analysis of the biologically treated sewage effluent (BTSE) by different treatment (initial DOC = 12–13 mg/L; TiO2 dose = 1.0 g/L; UV light intensity = 46.61 mW/cm2 ; radiation duration = 3 h; filtration flux = 20 L/m2 h).
oxidation and the submerged membrane reactor was able to operate at a constant flux for 5 h with a backwash frequency of 1 h. Although critical filtration flux observed after flocculation and photocatalysis also remained at 40 L/m2 h (same flux as that with a pre-treatment of photocatalysis only), the transmembrane pressure (TMP) of the submerged module was found to be comparatively low (Fig. 10). 3.4. Performance of the integrated photocatalysis and membrane system The synergic effects of the integrated photocatalysis— membrane system with pre-flocculation were evaluated in terms of organic removal efficiency (Fig. 11). Membrane filtration alone was able to remove about 20% of dissolved organics at a low filtration flux of 20 L/m2 h. When it was coupled with photocatalytic oxidation, the organic degradation efficiency of the integrated system and the filtration flux of the membrane reactor were doubled. Membrane fouling was significantly reduced and the performance was enhanced. Pre-flocculation with 50 mg/L of FeCl3 was found to effectively facilitate the degradation of organic matter by the photocatalytic oxidation process. The TOC removal of the hybrid system increased to 80%. The molecular weight distribution of organic matter after the pre-treatment was measured using high performance liquid chromatography (HPLC) analysis. Besides the major peak at the MW of 1500–2000 Da, there emerged another apparent peak at 100 Da and a minor one at 900 Da (Fig. 12). As can be seen from Fig. 10, membrane filtration and photocatalysis were able to remove large molecules rather than small ones. However, when pre-flocculation was applied, the absorbance intensities of these peaks reduced significantly. It indicated that pre-flocculation effectively enhanced the degradation of large and small molecular weight fractions. Moreover, no by-products were detected by the HPLC. 4. Conclusions The photocatalytic oxidation of organic compounds in BTSE was studied at varying TiO2 amount and other operating conditions. Pre-treatment by flocculation and membrane filtration were also employed in the treatment system. Several conclusions made from this study are as follows:
• The photocatalytic oxidation process was significantly affected by catalyst doses and only marginally by the detention time. Higher catalyst doses were able to facilitate more radicals and faster reactions to occur. At the optimal operating conditions, photocatalysis by P25/UV irradiation yielded approximately 42% of DOC removal efficiency after 120 min. • Pre-treatment by flocculation was found to enhance the degradation of organic compounds and improve the effluent quality. The removal efficiency was increased by 40% after flocculation, thus providing a total efficiency up to 80%. However, the pH readings of treated water ranged between 5.5 and 6.5, which were lower than the limit of discharged water. Therefore, another step to adjust pH of the effluent is required that may raise in the overall cost. • The critical flux of the submerged membrane increased from 25 to 40 L/m2 h after a pre-treatment of photocatalysis. Prephotooxidation with TiO2 /UV could effectively reduce membrane fouling and enhance the critical flux of the submerged membrane reactor. In addition, the HPLC analysis demonstrated that the integration of photocatalysis and microfiltration was able to remove both large (1500–2000 Da) and small MW organics (100–200 Da). Acknowledgement This research was funded by Australian Research Council (ARC) Discovery Grant DP0666257. References [1] S. Parsons, Advanced oxidation processes in water and wastewater treatment, IWA Publishing, London, 2004. [2] H.K. Shon, S. Vigneswaran, H.H. Ngo, J.H. Kim, Chemical coupling of photocatalysis with flocculation and adsorption in the removal of organic matter, Wat. Res. 39 (2005) 2549–2558. [3] D.S. Bhatkhande, V.G. Pangarkar, A. Beenackers, Photocatalytic degradation for environmental applications—a review, Chem. Technol. Biotechnol. 77 (2001) 102–116. [4] J. Arana, J.A.H. Melian, J.M.D. Rodriguez, O.G. Diaz, A. Viera, J.P. Pena, P.M.M. Sosa, V.E. Jimenez, TiO2 -photocatalysis as a tertiary treatment of naturally treated wastewater, Catal. Today 76 (2002) 279–289. [5] R. Pelton, X. Geng, M. Brook, Photocatalytic paper from colloidal TiO2 —fact of fantasy, Adv. Colloid Interf. Sci. 127 (2006) 42–53. [6] S. Mozia, A.W. Morawski, Hybridization of photocatalysis and membrane distillation for purification of wastewater, Catal. Today 118 (2006) 181–188. [7] Sydney Olympic Park Authority (SOPA), Review of Water and Wastewater Industry Structure and Pricing in the Greater Sydney Metropolitan Area, 2005, http://www.ipart.nsw.gov.au/. [8] J. Kleine, K.V. Peinemann, C. Schuster, H.J. Warnecke, Multifunctional system for treatment of wastewaters from adhesive-producing industries: separation
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