This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

Transcription

1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Desalination 247 (2009) Enhanced or reduced concentration polarization by membrane fouling in seawater reverse osmosis (SWRO) processes Suhan Kim a, Sungyun Lee b, Eunkyung Lee b, Sarp Sarper b, Chung-Hwan Kim a, Jaeweon Cho b, * a Water Research Center, Korea Institute of Water and Environment, Korea Water Resources Corporation (K-water), Jeonmin-Dong, Yuseong-Gu, Daejeon , Korea b Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju , Korea Tel , Fax: ; jwcho@gist.ac.kr Received 13 November 2008; revised 17 December 2008; accepted 24 December 2008 Abstract Salt rejection behaviors coupled with membrane fouling in seawater reverse osmosis (SWRO) processes were studied through a lab-scale membrane fouling test. Seawater was taken from Masan Bay, South Korea and foulants such as humic acid (HA), sodium alginate (SA), and silica nanoparticle (S120; particle diameter = 120 nm), were spiked to the seawater for the rapid fouling test. All the test conditions except model foulant were controlled equally. Salt concentration in permeate increases with flux decline even with no fouling because of increased water permeation and relatively constant salt passage. Salt mass transfer can be altered in the presence of fouling layer. Organic foulants such as HA and SA tend to build up denser fouling layer than S120 nanoparticles. Thus, S120 fouling layer induced cake enhanced concentration polarization (CECP) to accelerate salt rejection decreasing rate while HA and SA fouling layer hindered the convection of salt to lead cake reduced concentration polarization (CRCP) decelerating the rate. These combined effects of flux decline, CECP, and CRCP can be the causes of the coupled behaviors of permeate flux and salt rejection. Keywords: Seawater reverse osmosis (SWRO); Fouling; Salt rejection; Cake enhanced concentration polarization (CECP); Cake reduced concentration polarization (CRCP) *Corresponding author. Presented at the 2nd joint workshop between the Center for Seawater Desalination Plant and the European Desalination Society, Gwangju Institute of Science and Technology, Korea, October 8 9, /09/$ See front matter 2009 Published by Elsevier B.V. doi: /j.desal

3 S. Kim et al. / Desalination 247 (2009) Introduction Recently many countries, especially in Middle East, rely on seawater desalination to meet their fresh water requirements. Besides it is expected that the more countries will obtain freshwater by desalination in the future as a result of the rise in population rates in the world [1]. Various desalination technologies have been developed over the years such as thermal distillation, membrane, freezing and electrodialysis, etc. Among theses desalination technologies, reverse osmosis processes has been shown a gradual increase and are expected to be dominant in the future due to its lower cost and simplicity. The reverse osmosis technologies can promise to supply high quality fresh water by removing not only salinity but also toxic element, i.e. boron, from seawater [2]. Membrane fouling, however, is a major obstacle for operation and cost efficiency of seawater reverse osmosis (SWRO) process. The salt removal efficiency has a relationship with permeate flow rate as described by models based on transport parameter (i.e., mass transfer coefficients and permeability coefficients of water and salts) in RO system [3]. Generally, salt concentration in permeate increases as the permeate flux decrease because of less water permeation and relatively constant salt passage. Thus, reduction of permeate flow by membrane fouling can decrease salt removal efficiency in desalination process. But there exists another effect of fouling on salt rejection. It is the interaction between salt concentration polarization layer and fouling layer. Unfortunately, the effect of the fouling on salt rejection has not been clearly unders tood due to the variety of fouling such as inorganic and organic colloidal fouling, dissolved organic matter induced fouling, and bio-fouling with different characteristics. Con troversial effects of the membrane fouling on salt rejection can be expected depending on types of fouling. For the effect of colloidal fouling on salt rejection, Hoek and Elimelech [4] suggested cake-enhanced concentration polarization describing hindrance of the back diffusion of accumulated salt near membrane by colloidal fouling layer, subsequently reducing salt rejection. Similar reduction results were found by other researchers who conducted experiments using iron hydroxide [5] and silica colloids [6,7]. However, less information is available about the effect of the organic matter fouling on performance of SWRO membrane in terms of salt rejection despite of the importance of organic matter fouling in membrane operation. Lipp et al. [5] reported that salt rejection using RO membrane improved by humic substance fouling layer. Meanwhile, other studies reported that organic fouling can increase or decrease salt rejection depending on membrane characteristics [8,9]. There have been few studies on fouling coupled with salt rejection in RO processes using seawater. In this study, we investigated the effect of fouling on salt rejection by SWRO membrane using representative model foulants spiked in a real seawater sample. For organic foulants, sodium alginate and humic acid were used as representative model for polysaccharides of extracellular polymeric substances (EPS) and natural organic matters (NOM), respectively [10]. In addition, silica nanoparticles were used as inorganic colloidal foulant [11]. 2. Materials and methods 2.1. Membrane, seawater, and foulants A pilot sample of SWRO membrane was used in this study. The zeta potential value of the membrane was determined from electrophoretic mobility measurements using ELS-8000 apparatus produced by Otsuka

4 164 S. Kim et al. / Desalination 247 (2009) Electronics, Japan. Contact angle value of the membrane was determined by using sessile drop method. Roughness of the membrane was determined by using atomic force microscope (AFM) and scanning electron microscope (SEM). In the pore size distribution measurements polyethylene glycol (PEG) solution was prepared from PEG which has 200 Da of molecular weight then this solution was filtered by membranes. Feed and permeate samples were taken in order to investigate molecular weight cut off (MWCO) and removal efficiencies. Samples were analyzed in HPLC SEC apparatus. Seawater sample was taken from Masan Bay, South Korea. Dissolved organic carbon (DOC) concentration and ph of the seawater is 2 mg/l and 8.1, respectively. Sodium alginate (SA) and humic acid (HA) from Aldrich, were used as model organic foulants and spherical silica nanoparticles with 120 nm of average diameter (S120) from Nissan Chemical, were used as model inorganic colloidal foulant. The spiking concentration of SA, HA, and S120 were 75 mg/l as DOC, 960 mg/l as DOC, and 500 mg/l, respectively, which are enough to ignore the effect of original foulant contained in the seawater sample for a short period (5 h) fouling test Membrane fouling test The crossflow membrane filter (CMF) used in fouling tests was a modified version of a commercially available unit (Sepa CF, Osmonics, Inc.; Minnetonka, MN). Membrane surface area was m 2 and cross-sectional flow area was m 2. The fouling experiments were performed with full re-circulation (permeate and retentate recycled to the feed tank). Before each experiment, de-ionized water was filtered through the membrane prior to the fouling experiment for h to stabilize the filtration system. After stable flux was achieved, membrane resistance, salt rejection, and osmotiressure drop were measured following the procedure described elsewhere [12]. Temperature was maintained at 25 C by a recirculating chiller. The operation was conducted at constant pressure of 5.9 MPa and cross-flow velocity of 5 cm/s. The permeate flux were recorded by a digital flowmeter. 3. Results and discussion 3.1. Membrane characteristics The RO membrane tested in this study is a thin film composite membrane with polyamide. Membrane hydraulic resistance (R m ) and contact angle with de-ionized water is around m 1 and 57 ± 2, respectively. Zeta potential of the membrane in KCl 10 mm solution is 30 to 40 mv at ph range 6 8 and the roughness is measured as 61 nm. The SEM and AFM images of the membrane can be seen elsewhere [2] Relationship between flux and salt rejection In principle the permeate velocity affects salt rejection in RO systems, which can be described by models based on transport parameter. Salt concentration in permeate increases as the permeate flux decrease because of less water permeation and relatively constant salt passage. Thus, reduction of permeate flow by membrane fouling can decrease salt removal efficiency in desalination process as shown in Fig. 1, which shows a relationship between salt rejection and permeate velocity in this study. In the salt rejection experiments, trans-membrane pressure varied from 2.8 to 5.9 MPa to change permeate flux without

5 S. Kim et al. / Desalination 247 (2009) Experiment Regression this is a limitation of lab-scale filtration test for SWRO systems. (%) = 0.03 ln(ν w ) R 2 = ν w (μm/s) Fig. 1. Relationship between salt rejection ( ) and permeate velocity (v w ) without fouling. injecting foulants. It can be observed that salt rejection increases as permeate flux increases. An interesting feature in Fig. 1 is that salt rejecting ability of the tested membrane ( < 98%) is not good enough for SWRO membrane ( > 99%). There are a couple of reasons to explain this phenomenon. First one is that the tested membrane is still being developed. Second, the testing condition was not set to fit SWRO operation requirements. The tests were carried out at a low cross-flow velocity of 5 cm/s without feed channel spacer, which resulted in higher concentration polarization and osmotiressure drop to decrease permeate flux and increase salt passage. At transmembrane pressure of 5.9 MPa, the osmotic pressure drop calculated using the procedure described elsewhere [12] is 40 50% higher than the seawater osmotiressure. In field applications of RO process, the osmotiressure is just 10 20% higher than the feed osmotic pressure because of high cross-flow velocity (i.e cm/s) and help of feed spacer to make turbulence in cross-flow. Since seawater osmotiressure is around 3.0 MPa, the osmotiressure difference of 30% will be MPa and it is big enough to inhibit salt rejection of SWRO membranes. Unfortunately 3.3. Fouling coupled with salt rejection Figure 2 shows fouling test results using S120, HA, and SA as model foulants for inorganic colloidal fouling, NOM fouling, and fouling by polysaccharides of EPS, respectively. Since highly concentrated foulants were injected into the real seawater for the tests, sharp flux decline behaviors were observed in a range of 34 47% for 5 h of filtration at a constant pressure of 5.9 MPa. The highest salt passage is observed in the case of S120 fouling. The started to increase from the start-up of the operation and reached up to 35% increased value over,0, which is the highest of all the cases. This could be the coupled effect of flux decline and cake enhanced concentration polarization (CECP) phenomenon suggested by Hoek and Elimelech [4]. In case of HA and SA fouling tests, the started to decrease from the start-up of the operation and then increase with time. The hindrance of salt passage by the organic fouling layer could be the reason for the first increase of salt rejection. The initial decreasing rate was higher in case of SA than HA, which means polysaccharide makes denser fouling layer than NOM. For this reason the larger flux decline was observed in case of SA than HA although the concentration of SA is less than 10% of HA (75 and 960 mg/l as DOC for SA and HA, respectively). We call this phenomenon as cake reduced concentration polarization (CRCP) as a match for CECP. The combined effects of flux decline, CECP, and CRCP could be the causes of the coupled behaviors of permeate flux and salt rejection as shown in Fig. 2. In the presence of fouling cake layer, the salt mass transfer can be altered by the cake

6 166 S. Kim et al. / Desalination 247 (2009) (a) (b) ν p /ν p,0, 0.7 ν w /ν w, t, min ν p /ν p,0, 0.7 ν w /ν w, t, min (c) ν p /ν p,0, ν w /ν w, t, min Fig. 2. Fouling test results: normalized permeation velocity (v w /v w,0 ), salt rejection ( ), and normalized salt concentration in permeate ( ). (a) Inorganic colloidal fouling by S120. (b) Organic fouling by HA. (c) Organic fouling by SA. layer. The cake layer structure can hinder diffusion or convection of salt. When hindered diffusion mechanism is dominant, CECP occurs to increase salt passage while CRCP appears as hindered convection mechanism becomes dominant as described in Fig. 3. Salt rejection data as shown in Fig. 2 can be expressed as a function of flux decline, CECP, and CRCP. By using the relationship between permeate flux and salt rejection without fouling in Fig. 2, the effects of flux decline and fouling layer (i.e., CECP and CRCP) can be divided. Figure 4 shows the pure effect of fouling layer on the salt rejection and Δ f is the difference between observed salt rejection,, and estimated salt rejection using the regression equation in Fig. 1. The former is resulted from the effect of flux decline and fouling layer while the latter comes from the effect of flux decline only. Therefore Δ f can be expressed as Cake Layer C p2 C p0 C p1 C b C c Fig. 3. Concept of CECP and CRCP: C m1 > C m0 > C m2 and C p1 > C p0 > C p2. C m2 C m0 C m1 C m0, C p0 - Hindered convection = Hindered diffusion C m1, C p1 - Hindered diffusion is dominant [CECP] C m2, C p2 - Hindered convection is dominant [CRCP] CP Layer Membrane

7 S. Kim et al. / Desalination 247 (2009) Δ f (%) t, min HA SA S120 Fig. 4. The effect of fouling layer on the salt rejection. -4 Δ f R0 = R0 ( 003. ln( vw ) ) (1) where v w is permeate velocity in μm/s. The positive values of Δ f shown in organic fouling tests mean CRCP is dominant while the negative values of Δ f in inorganic fouling tests mean CECP is dominnt. 4. Conclusions Fouling affects salt rejection in seawater reverse osmosis (SWRO) processes with two different mechanisms. First, salt concentration in permeate generally increases with flux decline because pure water flux decline rate is higher than salt flux decline rate. Second, salt mass transfer can be altered inside the fouling layer structure to result in cake enhanced concentration polarization (CECP) or cake reduced concentration polarization (CRCP). Denser fouling layers tend to induce cake reduced concentration polarization (CRCP) rather than cake enhanced concentration polarization (CECP). Organic foulants used in this study tend to build up denser fouling layer than nanoparticles. As a result, nanoparticle fouling layer induced CECP to accelerate salt rejection decreasing rate while organic fouling layer hindered the convection of salt to lead CRCP decelerating the rate. Between the model organic foulants for polysaccharides of extracellular polymeric substances (EPS) and natural organic matters (NOM), the former makes denser fouling layer than the latter. These combined effects of flux decline, CECP, and CRCP can be the causes of the coupled behaviors of permeate flux and salt rejection in SWRO processes. Acknowledgement This research was supported by a grant from the National Research Laboratory Program by the Korea Science and Engineering Foundation (NOM Ecology Lab: ). References [1] A.D. Khawaji, I.L. Kutubkhanah and J.-M. Wie, Advances in seawater desalination technologies, Desalination, 221 (2008) [2] S. Sarp, S. Lee, X. Ren, E. Lee, K. Chon, S.H. Choi, S. Kim, I.S. Kim and J. Cho, Boron removal from seawater using NF and RO membranes, and effects of boron on HEK 293 human embryonic kidney cell with respect to toxicities, Desalination, 223 (2008)

8 168 S. Kim et al. / Desalination 247 (2009) [3] N.G. Voros, Z.B. Maroulis and D. Marinos- Kouris, Salt and water permeability in reverse osmosis membranes, Desalination, 104 (1996) [4] E.M.V. Hoek and M. Elimelech, Cake-enhanced concentration polarization: a new fouling mechanism for salt-rejecting membranes, Environ. Sci. Technol., 37 (2003) [5] P. Lipp, R. Gimbel and F.H. Frimmel, Parameters influencing the rejection properties of FT30 membranes, J. Membr. Sci., 95 (1994) [6] S. Lee, J. Cho and M. Elimelech, Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes, Desalination, 160 (2004) [7] H.Y. Ng, RO membrane solute rejection behavior at the initial stage of colloidal fouling, Desalination, 174 (2005) [8] L.D. Nghiem and S. Hawkes, Effects of membrane fouling on the nanofiltration of pharmaceutically active compounds (PhACs): mechanisms and role of membrane pore size, Sep. Purif. Technol., 57 (2007) [9] K.O. Agenson and T. Urase, Change in membrane performance due to organic fouling in nano filtration (NF)/reverse osmosis (RO) applications, Sep. Purif. Technol., 55 (2007) [10] H. Susanto, H. Arafat, E.M.L. Janssen and M. Ulbricht, Ultrafiltration of polysaccharide protein mixtures: elucidation of fouling mechanisms and fouling control by membrane surface modification, Sep. Purif. Technol., 63 (2008) [11] L.D. Nghiem and P.J. Coleman, NF/RO filtration of the hydrophobic ionogenic compound triclosan: transport mechanisms and the influence of membrane fouling, Sep. Purif. Technol., 62 (2008) [12] S. Kim and E.M.V. Hoek, Modeling concentration polarization in reverse osmosis processes, Desalination 186 (2006)