- Email: [email protected]
Performance evaluation of pretreatment processes in integrated membrane system for wastewater reuse
Desalination 250 (2010) 673–676
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Performance evaluation of pretreatment processes in integrated membrane system for wastewater reuse☆ Chanhyuk Park a, Seok-Won Hong a,b, Tai Hak Chung b, Yong-Su Choi a,⁎ a b
Center for Environmental Technology Research, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea Department of Civil, Urban and Geo-Systems Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history: Accepted 11 March 2009 Available online 8 November 2009 Keywords: Coagulation Microfiltration Ozonation Pretreatment Wastewater reuse
a b s t r a c t A series of lab-scale filtration experiments were performed under various operating conditions to investigate the fouling behavior of microfiltration (MF) membranes when employing two different pretreatment methods. The secondary effluents from a biologically advanced treatment process were fed to each hybrid system, consisting of coagulation–flocculation–MF (CF–MF) and ozonation–MF processes. All experiments were carried out using a stirred-cell system, which consisted of polyvinylidene difluoride (PVDF) MF membranes with a 0.22 μm pore size. When MF membrane was used alone without any pretreatment, the permeate flux dropped significantly. However, in the case of employing polyaluminium chloride (PACl) coagulation and ozonation as a pretreatment, the extent of flux decline rates was enhanced up to 88 and 38%, respectively. In the CF–MF hybrid system, the removal efficiencies of COD and total phosphorus were significantly enhanced at a coagulant dose above 30 mg/L. With ozonation, more than 90% of the color was removed even at a low dosage of ozone (5 mg/L). Therefore, ozonation would be strongly recommended as a pretreatment in terms of removing organic matter. The permeate water quality by ozonation–MF process was in good compliance with the guidelines for wastewater reuse proposed by South Korean Ministry of Environment. © 2009 Published by Elsevier B.V.
1 . Introduction Water resources are becoming increasingly scarce in many areas of the world due to development and increased demand [1,2]. It is not easy to develop new sources of water supply, thus, wastewater reclamation and reuse may play an important role in the development of strategies for the utilization of water resources [3]. Especially, wastewater reuse may reinforce water savings generating supplementary water resources, which are important in areas with limited rainfall [4]. In Korea, municipal wastewaters are commonly treated by conventional activated sludge systems that use suspended microbes to remove organics and nutrients, and large clarification tanks to separate the solid and liquid fractions. The most common processes used for wastewater reuse are filtration and adsorption, followed by disinfection using chlorine or ozone. Although numerous advanced wastewater technologies have been proposed for the production of effluents with a water quality complying with the specific applications of wastewater reuse, microfiltration (MF) membranes are gaining popularity in the world [5,6]. This is because those membrane filtrations can meet the regulations and guidelines associated with reclaimed water and produce constant quality effluents, which are not ☆ Presented at the Conference on Membranes in Drinking and Industrial Water Production, 20–24 October 2008, Toulouse, France. ⁎ Corresponding author. Tel.: +82 2 958 5834; fax: +82 2 958 5839. E-mail address: [email protected] (Y.-S. Choi). 0011-9164/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.03.023
affected by the performance of the activated sludge system and by eventual variations of wastewater effluent quality [7]. However, the effluent water from the activated sludge process still contains dissolved species and particulate substances that act to foul the membranes of the subsequent MF system used as a final barrier to contaminants in the product water [8,9]. Especially, organics are not able to be removed by filtration, even if the membrane is coupled with coagulation–flocculation process [10,11]. The residual dissolved organic matter of treated wastewaters may be effectively removed by the application of adsorption, ozonation or high-pressure membranes [12]. Therefore, these combinations of MF membranes with any other physical and chemical processes as a pretreatment make it possible to enhance their treatment quality and to mitigate the membrane fouling [13–15]. The main objectives of this study were to evaluate the effect of two different pretreatment processes (coagulation–flocculation (CF) and ozonation) on the performance of MF membranes during the reclamation of biologically advanced treated wastewater. Specifically, feed and permeate water quality of the two pretreatments and MF system were evaluated by measuring concentrations of solid (turbidity) and organic (UV254, DOC and color) parameters under various operating conditions. In addition, the permeate flux decline experiments were carried out to optimize the chemical coagulant and ozone dosages which can reduce the fouling potential of MF membranes. This work was delineated for future process optimization of the MF membrane with any pretreatment system.
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2. Materials and methods
2.4. Analytical methods
2.1. Source water The source water used in this study was a secondary treated wastewater obtained from the newly developed biological wastewater treatment pilot plant located in Osan city, Korea. This facility treats municipal wastewater using a new media containing activators for microorganisms in aeration and consists of the following unit operations: primary sedimentation, anoxic/aerobic, and secondary sedimentation. Source water for the current research was obtained immediately after secondary sedimentation. The advanced wastewater treatment plant was designed to treat about 50 m3/d of municipal wastewaters for removal of suspended solids and nutrients. Table 1 presents source water characteristics measured during the operation of the pilot plant. As shown, low turbidity and suspended solids were observed primarily due to sedimentation process. Moreover, the UV254 and DOC were fairly consistent and typical of advanced wastewater effluent. Finally, the nutrients of this wastewater were effectively treated and offered adequate feed concentrations to assess the impact of different types of pretreatment on MF membrane during this study.
A complete water quality analysis was performed during each filtration experiment to investigate the performance of the MF membranes at retaining solid and organic foulants present in the feed water. All solutions were prepared with ACS grade (Fisher Scientific, Pittsburgh, PA) and were used without further purification. Chemical solutions and feed waters for the fouling experiments were prepared with deionized (DI) water (Milli-Q® Ultrapure Water Purification System, Millipore Corp., Billerica, MA) having a conductivity less than 0.8 μS/cm when in equilibrium with atmospheric CO2. During all of the fouling experiments, feed and permeate samples were collected and analyzed for pH (Model 63, YSI Inc., Yellow Springs, Ohio). Two water quality parameters were measured to assess each foulant group as follows: turbidity for solids, UV254, DOC and color for organics. Turbidity was measured by a turbidimeter (2100P, Hach, USA). Concentrations of DOC were determined by a TOC analyzer (TOC-VCPH, Shimadzu, Japan) and UV254 absorbance was measured by a spectrophotometer (DR2000, Hach, USA). All these analytical concentrations such as CODcr, SS, T-N and T-P were measured according to Standard Methods for the Examination of Water and Wastewater [16].
2.2. Pretreatments
3. Results and discussion
Coagulation–flocculation (CF) experiments were performed in a conventional jar-test apparatus, equipped with six beakers of 1 L volume at room temperature. The experimental procedures consisted of the following steps: vigorous mixing (30 s), low mixing (15 min) and settling (20 min). The coagulant was a commercially obtained polyaluminium chloride (Flopac 41, Nalco) and was added to the desired concentration at the beginning of the first step (vigorous mixing). In other pretreatment methods, ozonation is conducted to improve the removal of soluble matters which cannot be easily removed using MF membranes. Ozone gas was generated using an ozone generator (Model DXO-20, Daehwa Eng., Republic of Korea). Three different ozone doses were examined (0.5, 1.0 and 1.5 mg/L) in order to reduce the fouling potential of MF membranes and to enhance the water quality including UV254, DOC and color.
3.1. Performance evaluation of pretreatment methods
2.3. MF membrane system operation Secondary effluents were mixed with the PACl coagulant or ozonated solution and were fed to a MF membrane within dead-end stirred-cell apparatus. The dead-end stirred cell (Model 8200, Amicon Corp., USA) used in this study was made for experimental use on a laboratory scale. The membrane module consists of a flat-sheet type that has a nominal pore size of 0.22 μm and the effective surface area has 3×10− 3 m2. The MF membranes used are PVDF (polyvinylidene difluoride) which has a high tolerance to ozone exposure, therefore, de-ozonation treatment prior to MF membrane was not carried out. In each experiment, new membrane was used to obtain reproducible results. These experiments were conducted under ambient conditions at a temperature range of 20–25 °C.
The performance of CF process was observed to reduce the particulate fouling potential of feed water. In general, MF membranes were mainly used to remove the particulate matters, and particulates less than membrane pores and colloids were not effectively removed as reported in previous studies [17]. Therefore, the particulate fouling potential is remarkably increased when colloids or small particulates are dominant and pretreatment processes are not applied. Removal efficiency observed during the CF and ozonation process as pretreatment is depicted in Fig. 1. The reduction of turbidity was about 84.3% after CF, while the ozonation process decreased turbidity removal to more than 60%. The 254 nm UV absorption values may be considered as a rough indication of organic compounds contained in the reclaimed wastewater, mostly in the form of humic-like substances. The reduction of UV absorbance after CF did not exceed 9.5%, whereas the highest UV absorbance reduction, slightly exceeded 58.5%, and was observed at the outlet of ozonation unit. DOC removal rate was lower than 37.5% during CF, whereas the removal of organic matters results in a lower overall reduction of UV absorbance than DOC, indicating that organic substances of different
Table 1 Feed water quality analysis results. Parameter
Feed water (average ± S.D.)
pH CODcr (mg/L) Turbidity (NTU) SS (mg/L) UV254 (cm− 1) DOC (mg/L) Color (CU) T-P (mg/L) T-N (mg/L)
7.2 ± 0.61 35.0 ± 8.15 1.53 ± 1.82 7.1 ± 5.9 0.095 ± 0.021 6.29 ± 1.53 30 ± 4 2.98 ± 1.68 11.1 ± 3.4
Fig. 1. Highest removal efficiency observed during the CF and ozonation processes.
C. Park et al. / Desalination 250 (2010) 673–676
structure might retain after ozonation. The removal capacity of total phosphorus was completely removed during the addition of PACl coagulant, followed by CF, whereas the subsequent ozonation did not further enhance the reduction of total phosphorus. Total nitrogen was only partially removed by CF processes, slightly exceeding 10%, however, the reduction of total nitrogen concentrations by ozonation process was enhanced up to 44%.
3.2. Optimization of pretreatment methods for fouling control The pretreated water was fed to the MF membranes, where various PACl coagulant and ozone dosages were applied for the assessment of performance of MF membranes and the determination of optimum concentration supply. Typical removal efficiencies of turbidity, UV254 absorbance, DOC, T-N and T-P, are presented in Fig. 2(a) as a function of coagulant dosage. CF processes at the lowest dose, 30.0 mg/L, resulted in a turbidity reduction of about 84.3% and in a complete total phosphorus reduction. At the same conditions, UV254 and total nitrogen reduction were lower than 10 and 16%, respectively. Fig. 2(b) shows the removal efficiencies of those parameters as a function of coagulant dosage. The application of higher ozone doses resulted in a significant increase in turbidity removal, reaching up to 60.1%, at the higher ozone dosage, 15.0 mg/L, resulting in an effluent of high quality, with a turbidity of 0.61 NTU. The UV254 absorbance values
Fig. 2. Typical removal efficiencies during CF and ozonation of effluents; (a) a function of PACl dose, (b) a function of ozone dose.
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were significantly decreased by ozonation, at an ozone dosage of 15.0 mg/L, and the highest UV254 absorbance removal efficiency slightly exceeded 58.5%. The maximum DOC removal capacity exceeded 82.2% at an ozone dosage of 15.0 mg/L. The total phosphorus removal was not mostly affected by ozone dosage, unlike the CF process. However, ozonation was considered as an effective process for the removal of total nitrogen; the application of 5.0 mg/L ozone dose resulted in the highest total nitrogen reduction, up to 44%. When MF membrane was used alone without any pretreatment, the permeate flux dropped significantly. The permeate flux was found to enhance up to 88% in 30 h when no pretreatment was employed as shown in Fig. 3(a). The CF–MF hybrid system slowed down the permeate flux decline rate, only decreased to 31% with no backwashing. In case of ozonation–MF hybrid system, the permeate flux decline was higher observed than the CF–MF hybrid system as presented in Fig. 3(b). A difference of the permeate flux decline with increasing the coagulant dose is not largely observed. The optimum dose was around 30 mg/L (taking into account the permeate flux decline and turbidity removal efficiency). When ozone dose was increased from 5.0 to 15.0 mg/L, TOC removal efficiency increased from 56.4 to 58.5%. There was hardly any difference in the efficiency when the ozone dose was increased. Due to the similar permeate flux decline rate regardless of ozone dose, the optimum dose in this case was approximately 5.0 mg/L when considering the several
Fig. 3. The permeate flux decline observed during MF membrane operations; (a) CF–MF hybrid process, (b) ozone–MF hybrid process.
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Table 2 The treated water quality of hybrid system with coagulant and ozone doses. Dose COD Turbidity Color T-N T-P pH (mg/L) (mg/L) (NTU) (CU) (mg/L) (mg/L) Guidelines (USEPA and South Korea) Treated wastewater (feed water) MF CF–MF
Ozonation–MF
–
20
2
10
10
1
–
35
1.53
30
11.1
2.98
5.8 – 8.5 7.2
– 30 40 50 60 70 5 10 15
31 13 13 10 14 18 25 13 14
0.11 0.24 0.28 0.27 0.25 0.29 0.87 0.78 0.61
28 11 12 10 13 12 3 2 3
9.7 9.9 9.3 9.4 9.3 9.5 6.2 7.9 7.5
2.73 N.D N.D N.D N.D N.D 2.67 2.80 2.84
7.8 6.8 6.6 6.0 5.1 4.6 7.6 7.6 7.6
parameters such as UV254 and total nitrogen concentrations. The incorporation of ozone removed the majority of organics which were not removed by CF process. Therefore, pretreatment methods should be relatively chosen by considering the target fouling potential of feed water. This contributed to the significant reduction in MF membrane fouling for wastewater reuse. 3.3. Filtrate quality The feed water used in this study has a high turbidity, color, and total phosphorus concentration, therefore, the hybrid system should be considered to meet the guidelines for wastewater reuse proposed by USEPA and South Korean Ministry of Environment. Table 2 shows the general water quality parameters when using the MF membranes, CF– MF hybrid, and ozonation–MF hybrid system for secondary wastewater reuse. When the MF membranes are used alone, COD, color and total phosphorus values are not required due to size exclusion mechanisms. These observations indicated that MF membrane was very effective at rejecting particulate matters such as turbidity, independent of operating flux. Consequently, the retained particles and dissolved organic matters would impose significant fouling potential on the system. In the CF–MF hybrid system, CODcr and turbidity were significantly removed in dependence of coagulant dose, and total phosphorus was completely removed, however, color was slightly reduced because significant portions of dissolved matter passed through the MF membranes. On the contrary, CODcr and color were remained below the limitations of any operating conditions when using ozonation–MF hybrid system, however, turbidity was significantly reduced at higher ozone doses. 4. Conclusions The reclamation potential of secondary wastewater effluents was investigated in this work by the application of a pretreatment unit, including CF and ozonation. The efficiency of the pretreatment was measured by various physicochemical parameters and the operation capacity of each process was evaluated for the removal of general parameters proposed as wastewater reuse guidelines. The CF process resulted in the removal of turbidity and total phosphorus, while the
color, UV254 and total nitrogen concentrations were not substantially affected. The effect of ozonation on organic matter removal was studied by the application of different ozone doses, between 5.0 and 15.0 mg/L. Further removal of turbidity, up to 80%, and reduction of DOC to 82.1% were observed at the highest ozone dosage. However, the total phosphorus concentration was not significantly affected despite the application of high ozone doses. In addition, the permeate flux decline rates were improved by employing CF and ozonation process, while the permeate flux was significantly reduced when MF membrane was used alone without any pretreatment. Acknowledgements This research is supported by a grant (code # 071-042-087) from the Eco-Technopia 21 Project by Korean Ministry of Environment and in part by Seoul National University SIR Group of the BK21 Research Program funded by the Korean Ministry of Education and Human Resources Development. This work was also supported by the Korea Institute of Science and Technology institutional research programs. References [1] M. Petala, V. Tsiridis, P. Samaras, A. Zouboulis, G.P. Sakellaropoulos, Wastewater reclamation by advanced treatment of secondary effluents, Desalination 195 (2006) 109–118. [2] G.K. Pearce, UF/MF pre-treatment to RO in seawater and wastewater reuse applications: a comparison of energy costs, Desalination 222 (2008) 66–73. [3] M. Abdel-Jawad, S. Ebrahim, M. Al-Tabtabaei, S. Al-Shammari, Advanced technologies for municipal wastewater purification: technical and economic assessment, Desalination 124 (1999) 251–261. [4] S.-K. Yim, W.-Y. Ahn, G.-T. Kim, G.-W. Koh, J. Cho, S.-H. Kim, Pilot-scale evaluation of an integrated membrane system for domestic wastewater reuse on islands, Desalination 208 (2007) 113–124. [5] B. Jimenez, A.M. Chavez, A. Leyva, G. Tchobanoglous, Sand and synthetic medium filtration of advanced primary treatment effluent from Mexico City, Water Res. 34 (2) (2002) 473–480. [6] P. Xu, M.-L. Janex, P. Savoye, A. Cockx, V. Lazarova, Wastewater disinfection by ozone: main parameters for process design, Water Res. 36 (4) (2002) 1043–1055. [7] M.F. Hamoda, Sand filtration of wastewater for tertiary treatment and water reuse, Desalination 164 (2004) 203–211. [8] C. Park, H. Kim, S. Hong, S.-I. Choi, Variation and prediction of membrane fouling index under various feed water characteristics, J. Membr. Sci. 284 (2006) 248–254. [9] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [10] M.-H. Cho, C.-H. Lee, S. Lee, Effect of flocculation conditions on membrane permeability in coagulation–microfiltration, Desalination 191 (2006) 386–396. [11] C. Park, H. Kim, S. Hong, S. Lee, S.-I. Choi, Evaluation of organic matter fouling potential by membrane fouling index, Water Sci. Technol. 7 (5–6) (2007) 27–33. [12] S. Hong, M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes, J. Membr. Sci. 132 (1997) 159–181. [13] J. Decarolis, S. Hong, J. Taylor, Fouling behavior of a pilot scale inside-out hollow fiber UF membrane during dead-end filtration of tertiary wastewater, J. Membr. Sci. 191 (2001) 165–178. [14] W.S. Guo, S. Vigneswaran, H.H. Ngo, H. Chapman, Experimental investigation of adsorption–flocculation–microfiltration hybrid system in wastewater reuse, J. Membr. Sci. 242 (2004) 27–35. [15] H.-S. Kim, H. Katayama, S. Takizawa, S. Ohgaki, Development of a microfilter separation system coupled with a high dose of powdered activated carbon for advanced water treatment, Desalination 186 (2005) 215–226. [16] Standard Methods for the Examination of Water and Wastewater, (19Ed.), American Public Health Association/American Water Works Association/Water Environment Federation, Washington, D. C., 1995. [17] S. Hong, P. Krishna, C. Hobbs, D. Kim, J. Cho, Variations in backwash efficiency during colloidal filtration of hollow-fiber microfiltration membranes, Desalination 173 (2005) 257–268.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Performance evaluation of pretreatment processes in integrated membrane system for wastewater reuse☆ Chanhyuk Park a, Seok-Won Hong a,b, Tai Hak Chung b, Yong-Su Choi a,⁎ a b
Center for Environmental Technology Research, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea Department of Civil, Urban and Geo-Systems Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history: Accepted 11 March 2009 Available online 8 November 2009 Keywords: Coagulation Microfiltration Ozonation Pretreatment Wastewater reuse
a b s t r a c t A series of lab-scale filtration experiments were performed under various operating conditions to investigate the fouling behavior of microfiltration (MF) membranes when employing two different pretreatment methods. The secondary effluents from a biologically advanced treatment process were fed to each hybrid system, consisting of coagulation–flocculation–MF (CF–MF) and ozonation–MF processes. All experiments were carried out using a stirred-cell system, which consisted of polyvinylidene difluoride (PVDF) MF membranes with a 0.22 μm pore size. When MF membrane was used alone without any pretreatment, the permeate flux dropped significantly. However, in the case of employing polyaluminium chloride (PACl) coagulation and ozonation as a pretreatment, the extent of flux decline rates was enhanced up to 88 and 38%, respectively. In the CF–MF hybrid system, the removal efficiencies of COD and total phosphorus were significantly enhanced at a coagulant dose above 30 mg/L. With ozonation, more than 90% of the color was removed even at a low dosage of ozone (5 mg/L). Therefore, ozonation would be strongly recommended as a pretreatment in terms of removing organic matter. The permeate water quality by ozonation–MF process was in good compliance with the guidelines for wastewater reuse proposed by South Korean Ministry of Environment. © 2009 Published by Elsevier B.V.
1 . Introduction Water resources are becoming increasingly scarce in many areas of the world due to development and increased demand [1,2]. It is not easy to develop new sources of water supply, thus, wastewater reclamation and reuse may play an important role in the development of strategies for the utilization of water resources [3]. Especially, wastewater reuse may reinforce water savings generating supplementary water resources, which are important in areas with limited rainfall [4]. In Korea, municipal wastewaters are commonly treated by conventional activated sludge systems that use suspended microbes to remove organics and nutrients, and large clarification tanks to separate the solid and liquid fractions. The most common processes used for wastewater reuse are filtration and adsorption, followed by disinfection using chlorine or ozone. Although numerous advanced wastewater technologies have been proposed for the production of effluents with a water quality complying with the specific applications of wastewater reuse, microfiltration (MF) membranes are gaining popularity in the world [5,6]. This is because those membrane filtrations can meet the regulations and guidelines associated with reclaimed water and produce constant quality effluents, which are not ☆ Presented at the Conference on Membranes in Drinking and Industrial Water Production, 20–24 October 2008, Toulouse, France. ⁎ Corresponding author. Tel.: +82 2 958 5834; fax: +82 2 958 5839. E-mail address: [email protected] (Y.-S. Choi). 0011-9164/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.03.023
affected by the performance of the activated sludge system and by eventual variations of wastewater effluent quality [7]. However, the effluent water from the activated sludge process still contains dissolved species and particulate substances that act to foul the membranes of the subsequent MF system used as a final barrier to contaminants in the product water [8,9]. Especially, organics are not able to be removed by filtration, even if the membrane is coupled with coagulation–flocculation process [10,11]. The residual dissolved organic matter of treated wastewaters may be effectively removed by the application of adsorption, ozonation or high-pressure membranes [12]. Therefore, these combinations of MF membranes with any other physical and chemical processes as a pretreatment make it possible to enhance their treatment quality and to mitigate the membrane fouling [13–15]. The main objectives of this study were to evaluate the effect of two different pretreatment processes (coagulation–flocculation (CF) and ozonation) on the performance of MF membranes during the reclamation of biologically advanced treated wastewater. Specifically, feed and permeate water quality of the two pretreatments and MF system were evaluated by measuring concentrations of solid (turbidity) and organic (UV254, DOC and color) parameters under various operating conditions. In addition, the permeate flux decline experiments were carried out to optimize the chemical coagulant and ozone dosages which can reduce the fouling potential of MF membranes. This work was delineated for future process optimization of the MF membrane with any pretreatment system.
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C. Park et al. / Desalination 250 (2010) 673–676
2. Materials and methods
2.4. Analytical methods
2.1. Source water The source water used in this study was a secondary treated wastewater obtained from the newly developed biological wastewater treatment pilot plant located in Osan city, Korea. This facility treats municipal wastewater using a new media containing activators for microorganisms in aeration and consists of the following unit operations: primary sedimentation, anoxic/aerobic, and secondary sedimentation. Source water for the current research was obtained immediately after secondary sedimentation. The advanced wastewater treatment plant was designed to treat about 50 m3/d of municipal wastewaters for removal of suspended solids and nutrients. Table 1 presents source water characteristics measured during the operation of the pilot plant. As shown, low turbidity and suspended solids were observed primarily due to sedimentation process. Moreover, the UV254 and DOC were fairly consistent and typical of advanced wastewater effluent. Finally, the nutrients of this wastewater were effectively treated and offered adequate feed concentrations to assess the impact of different types of pretreatment on MF membrane during this study.
A complete water quality analysis was performed during each filtration experiment to investigate the performance of the MF membranes at retaining solid and organic foulants present in the feed water. All solutions were prepared with ACS grade (Fisher Scientific, Pittsburgh, PA) and were used without further purification. Chemical solutions and feed waters for the fouling experiments were prepared with deionized (DI) water (Milli-Q® Ultrapure Water Purification System, Millipore Corp., Billerica, MA) having a conductivity less than 0.8 μS/cm when in equilibrium with atmospheric CO2. During all of the fouling experiments, feed and permeate samples were collected and analyzed for pH (Model 63, YSI Inc., Yellow Springs, Ohio). Two water quality parameters were measured to assess each foulant group as follows: turbidity for solids, UV254, DOC and color for organics. Turbidity was measured by a turbidimeter (2100P, Hach, USA). Concentrations of DOC were determined by a TOC analyzer (TOC-VCPH, Shimadzu, Japan) and UV254 absorbance was measured by a spectrophotometer (DR2000, Hach, USA). All these analytical concentrations such as CODcr, SS, T-N and T-P were measured according to Standard Methods for the Examination of Water and Wastewater [16].
2.2. Pretreatments
3. Results and discussion
Coagulation–flocculation (CF) experiments were performed in a conventional jar-test apparatus, equipped with six beakers of 1 L volume at room temperature. The experimental procedures consisted of the following steps: vigorous mixing (30 s), low mixing (15 min) and settling (20 min). The coagulant was a commercially obtained polyaluminium chloride (Flopac 41, Nalco) and was added to the desired concentration at the beginning of the first step (vigorous mixing). In other pretreatment methods, ozonation is conducted to improve the removal of soluble matters which cannot be easily removed using MF membranes. Ozone gas was generated using an ozone generator (Model DXO-20, Daehwa Eng., Republic of Korea). Three different ozone doses were examined (0.5, 1.0 and 1.5 mg/L) in order to reduce the fouling potential of MF membranes and to enhance the water quality including UV254, DOC and color.
3.1. Performance evaluation of pretreatment methods
2.3. MF membrane system operation Secondary effluents were mixed with the PACl coagulant or ozonated solution and were fed to a MF membrane within dead-end stirred-cell apparatus. The dead-end stirred cell (Model 8200, Amicon Corp., USA) used in this study was made for experimental use on a laboratory scale. The membrane module consists of a flat-sheet type that has a nominal pore size of 0.22 μm and the effective surface area has 3×10− 3 m2. The MF membranes used are PVDF (polyvinylidene difluoride) which has a high tolerance to ozone exposure, therefore, de-ozonation treatment prior to MF membrane was not carried out. In each experiment, new membrane was used to obtain reproducible results. These experiments were conducted under ambient conditions at a temperature range of 20–25 °C.
The performance of CF process was observed to reduce the particulate fouling potential of feed water. In general, MF membranes were mainly used to remove the particulate matters, and particulates less than membrane pores and colloids were not effectively removed as reported in previous studies [17]. Therefore, the particulate fouling potential is remarkably increased when colloids or small particulates are dominant and pretreatment processes are not applied. Removal efficiency observed during the CF and ozonation process as pretreatment is depicted in Fig. 1. The reduction of turbidity was about 84.3% after CF, while the ozonation process decreased turbidity removal to more than 60%. The 254 nm UV absorption values may be considered as a rough indication of organic compounds contained in the reclaimed wastewater, mostly in the form of humic-like substances. The reduction of UV absorbance after CF did not exceed 9.5%, whereas the highest UV absorbance reduction, slightly exceeded 58.5%, and was observed at the outlet of ozonation unit. DOC removal rate was lower than 37.5% during CF, whereas the removal of organic matters results in a lower overall reduction of UV absorbance than DOC, indicating that organic substances of different
Table 1 Feed water quality analysis results. Parameter
Feed water (average ± S.D.)
pH CODcr (mg/L) Turbidity (NTU) SS (mg/L) UV254 (cm− 1) DOC (mg/L) Color (CU) T-P (mg/L) T-N (mg/L)
7.2 ± 0.61 35.0 ± 8.15 1.53 ± 1.82 7.1 ± 5.9 0.095 ± 0.021 6.29 ± 1.53 30 ± 4 2.98 ± 1.68 11.1 ± 3.4
Fig. 1. Highest removal efficiency observed during the CF and ozonation processes.
C. Park et al. / Desalination 250 (2010) 673–676
structure might retain after ozonation. The removal capacity of total phosphorus was completely removed during the addition of PACl coagulant, followed by CF, whereas the subsequent ozonation did not further enhance the reduction of total phosphorus. Total nitrogen was only partially removed by CF processes, slightly exceeding 10%, however, the reduction of total nitrogen concentrations by ozonation process was enhanced up to 44%.
3.2. Optimization of pretreatment methods for fouling control The pretreated water was fed to the MF membranes, where various PACl coagulant and ozone dosages were applied for the assessment of performance of MF membranes and the determination of optimum concentration supply. Typical removal efficiencies of turbidity, UV254 absorbance, DOC, T-N and T-P, are presented in Fig. 2(a) as a function of coagulant dosage. CF processes at the lowest dose, 30.0 mg/L, resulted in a turbidity reduction of about 84.3% and in a complete total phosphorus reduction. At the same conditions, UV254 and total nitrogen reduction were lower than 10 and 16%, respectively. Fig. 2(b) shows the removal efficiencies of those parameters as a function of coagulant dosage. The application of higher ozone doses resulted in a significant increase in turbidity removal, reaching up to 60.1%, at the higher ozone dosage, 15.0 mg/L, resulting in an effluent of high quality, with a turbidity of 0.61 NTU. The UV254 absorbance values
Fig. 2. Typical removal efficiencies during CF and ozonation of effluents; (a) a function of PACl dose, (b) a function of ozone dose.
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were significantly decreased by ozonation, at an ozone dosage of 15.0 mg/L, and the highest UV254 absorbance removal efficiency slightly exceeded 58.5%. The maximum DOC removal capacity exceeded 82.2% at an ozone dosage of 15.0 mg/L. The total phosphorus removal was not mostly affected by ozone dosage, unlike the CF process. However, ozonation was considered as an effective process for the removal of total nitrogen; the application of 5.0 mg/L ozone dose resulted in the highest total nitrogen reduction, up to 44%. When MF membrane was used alone without any pretreatment, the permeate flux dropped significantly. The permeate flux was found to enhance up to 88% in 30 h when no pretreatment was employed as shown in Fig. 3(a). The CF–MF hybrid system slowed down the permeate flux decline rate, only decreased to 31% with no backwashing. In case of ozonation–MF hybrid system, the permeate flux decline was higher observed than the CF–MF hybrid system as presented in Fig. 3(b). A difference of the permeate flux decline with increasing the coagulant dose is not largely observed. The optimum dose was around 30 mg/L (taking into account the permeate flux decline and turbidity removal efficiency). When ozone dose was increased from 5.0 to 15.0 mg/L, TOC removal efficiency increased from 56.4 to 58.5%. There was hardly any difference in the efficiency when the ozone dose was increased. Due to the similar permeate flux decline rate regardless of ozone dose, the optimum dose in this case was approximately 5.0 mg/L when considering the several
Fig. 3. The permeate flux decline observed during MF membrane operations; (a) CF–MF hybrid process, (b) ozone–MF hybrid process.
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Table 2 The treated water quality of hybrid system with coagulant and ozone doses. Dose COD Turbidity Color T-N T-P pH (mg/L) (mg/L) (NTU) (CU) (mg/L) (mg/L) Guidelines (USEPA and South Korea) Treated wastewater (feed water) MF CF–MF
Ozonation–MF
–
20
2
10
10
1
–
35
1.53
30
11.1
2.98
5.8 – 8.5 7.2
– 30 40 50 60 70 5 10 15
31 13 13 10 14 18 25 13 14
0.11 0.24 0.28 0.27 0.25 0.29 0.87 0.78 0.61
28 11 12 10 13 12 3 2 3
9.7 9.9 9.3 9.4 9.3 9.5 6.2 7.9 7.5
2.73 N.D N.D N.D N.D N.D 2.67 2.80 2.84
7.8 6.8 6.6 6.0 5.1 4.6 7.6 7.6 7.6
parameters such as UV254 and total nitrogen concentrations. The incorporation of ozone removed the majority of organics which were not removed by CF process. Therefore, pretreatment methods should be relatively chosen by considering the target fouling potential of feed water. This contributed to the significant reduction in MF membrane fouling for wastewater reuse. 3.3. Filtrate quality The feed water used in this study has a high turbidity, color, and total phosphorus concentration, therefore, the hybrid system should be considered to meet the guidelines for wastewater reuse proposed by USEPA and South Korean Ministry of Environment. Table 2 shows the general water quality parameters when using the MF membranes, CF– MF hybrid, and ozonation–MF hybrid system for secondary wastewater reuse. When the MF membranes are used alone, COD, color and total phosphorus values are not required due to size exclusion mechanisms. These observations indicated that MF membrane was very effective at rejecting particulate matters such as turbidity, independent of operating flux. Consequently, the retained particles and dissolved organic matters would impose significant fouling potential on the system. In the CF–MF hybrid system, CODcr and turbidity were significantly removed in dependence of coagulant dose, and total phosphorus was completely removed, however, color was slightly reduced because significant portions of dissolved matter passed through the MF membranes. On the contrary, CODcr and color were remained below the limitations of any operating conditions when using ozonation–MF hybrid system, however, turbidity was significantly reduced at higher ozone doses. 4. Conclusions The reclamation potential of secondary wastewater effluents was investigated in this work by the application of a pretreatment unit, including CF and ozonation. The efficiency of the pretreatment was measured by various physicochemical parameters and the operation capacity of each process was evaluated for the removal of general parameters proposed as wastewater reuse guidelines. The CF process resulted in the removal of turbidity and total phosphorus, while the
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