An assessment of the Silt Density Index based on RO membrane ...

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236 S.G. Yiantsios, A. J. Karabelas /Desalination 151 (2002) 229-238 3.2. SDI measurements It is interesting to examine <strong>the</strong> predictions or recommendations that would be obtained by SD1 measurements if <strong>the</strong> same water were to be used as RO feed water. For this task water was prepared in <strong>the</strong> same way as for <strong>the</strong> fouling experiments and ferric ions were added at various concentrations. SD1 measurements were performed according to <strong>the</strong> standard conditions, that is, by use <strong>of</strong> 0.45 pm pore size Millipore filter disks at a pressure <strong>of</strong> 30 psi. At a concentration <strong>of</strong> 1 ppm almost complete plugging <strong>of</strong> <strong>the</strong> filter occurred within <strong>the</strong> first 2 min <strong>of</strong> <strong>the</strong> test. The upper limit <strong>of</strong> concentrations where measurable and meaningful SD1 values could be obtained is approximately 20 ppb, as can be seen from Fig. 5. Several interesting observations may be made by examination <strong>of</strong> <strong>the</strong> results in Figs. 4 and 5. First <strong>of</strong> all, it may be recalled that manufacturers’ recommendations for membrane feed waters under conditions favoring oxidation are that dissolved iron concentration should be less than 50 ppb. It may also be mentioned that fouling problems due to iron have been reported with NF membranes at even lower concentrations, and, <strong>the</strong>refore, this limit could be considered tentative [ 151. The upper limit <strong>of</strong> roughly 20 ppb for $3 2 1 .; + b f . / c 0 ~~.~_~.~~_..~~~~ _--t..“_-.-__._-.. _-.._I lFe3’~kW 15 20 Fig. 5. SD1 as a function <strong>of</strong> iron concentration in <strong>the</strong> feed water. precipitated iron obtained by <strong>the</strong> present SD1 measurements is not only remarkably close to manufacturers recommendations, but also on <strong>the</strong> correct side. A second observation is that <strong>the</strong>re is a notable sensitivity to particles much smaller that <strong>the</strong> pore size <strong>of</strong> <strong>the</strong> filter employed in <strong>the</strong> SD1 test. In this case, <strong>the</strong> observed filter fouling is more likely due to pore restriction and plugging ra<strong>the</strong>r than to cake formation. One might recall that a small change in pore diameter results in significant changes in pressure drop, <strong>the</strong> latter being proportional to pore diameter raised to <strong>the</strong> fourth power. Thus, despite <strong>the</strong> negligible particle retention, significant effects are observed due to pore restriction and plugging. Finally, from <strong>the</strong> linear relationship obtained in Fig. 4 on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> present RO fouling data, it may be projected that a similar membrane operating under similar conditions would exhibit a 6% drop in productivity within less than 45 or 30 days, if <strong>the</strong> feed water contained 10 ppb or 15 ppb <strong>of</strong> iron; <strong>the</strong>se concentrations correspond to SD1 values close to 3 or 4, respectively, which are within <strong>the</strong> range <strong>of</strong> recommended values. However, <strong>the</strong> above rates <strong>of</strong> flux decline are ra<strong>the</strong>r severe, and hence, under <strong>the</strong> conditions <strong>of</strong> <strong>the</strong> present tests, <strong>the</strong> SD1 does not appear to be conservative enough. Moreover, <strong>the</strong> SD1 cannot predict fouling rates, and, hence, it cannot discriminate between different types <strong>of</strong> membranes. For example, under <strong>the</strong> same conditions, an NF membrane would be expected to foul more rapidly and a seawater membrane to be less affected since <strong>the</strong>ir resistances are different compared to that <strong>of</strong> <strong>the</strong> fouling cake. It is, <strong>the</strong>refore, suggested that an improvement <strong>of</strong> <strong>the</strong> SDI, in <strong>the</strong> direction <strong>of</strong> predicting fouling rates and discriminating between various membranes is needed. 4. Conclusions In <strong>the</strong> present study iron oxide was selected as a typical inorganic colloidal foulant due to its
S.G. Yiantsios. A.J. Karabelas / Desalination 151 (2002) 229-238 237 importance, as evidenced from well known manufacturers’ recommendations on iron concentration in feed waters and from frequently encountered problems in membrane installations. Fouling data were obtained by continuous introduction <strong>of</strong> ferric ions in <strong>the</strong> feed water and spontaneous precipitation <strong>of</strong> colloidal iron hydrous oxide. Thus, <strong>the</strong> experimental procedure followed simulates phenomena taking place in actual membrane installations and only skips <strong>the</strong> oxidation step <strong>of</strong> dissolved ferrous ions. A range <strong>of</strong> iron concentrations was identified where a linear relationship existed between flux reduction rate and concentration under <strong>the</strong> experimental conditions employed. Although <strong>the</strong> results obtained represent accelerated fouling conditions, it is proposed that <strong>the</strong> linear relationship can be extrapolated to lower and more realistic concentrations. The upper limit <strong>of</strong> <strong>the</strong> concentration range where measurable and meaningful SD1 values could be obtained was roughly 20 ppb, which is remarkably close to membrane manufacturers’ recommendations. A notable sensitivity <strong>of</strong> <strong>the</strong> SD1 was also observed with particles for which retention is negligible, due mainly to filter pore restriction and plugging. However, on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> RO fouling data obtained, it may be projected that <strong>the</strong> SD1 is not conservative enough. Fur<strong>the</strong>rmore, since <strong>the</strong> SD1 cannot predict fouling rates, it cannot discriminate between different types <strong>of</strong> membranes (i.e., NF vs. high pressure RO). It is, <strong>the</strong>refore, suggested that an improvement <strong>of</strong> <strong>the</strong> SD1 is needed in <strong>the</strong> direction <strong>of</strong> predicting fouling rates and discriminating between various membrane types. Acknowledgments Financial support <strong>of</strong> this work by <strong>the</strong> Middle East Desalination Research Center (Project No 98-BS-034) is gratefully acknowledged. References 111 S.C.J.M. Van Ho<strong>of</strong>, J.G. Minnery and B. Mack, Performing a membrane autopsy, Desalination Water Reuse, ll(2002) 40-46. PI J.C. Schippers and J. Verdouw, The modified fouling index, a method <strong>of</strong> determining <strong>the</strong> fouling characteristics <strong>of</strong> water, Desalination, 32 (1980) 137-148. r31 S.F.E. Boerlage, M.D. Kennedy, M.R. Dickson, D.E.Y. El-Hodali and J.C. Schippers, The modified fouling index using ultrafiltration membranes (MFI- UF): characterization, filtration mechanisms and proposed reference membrane, J. Membr. Sci., 197 (2002) l-2 1. [41 S.F.E. Boerlage, M.D. Kennedy, M.P. <strong>An</strong>iye, E.M. Abogrean, G. Galjaard and J.C. Schippers, Monitoring particulate fouling in membrane systems, Desalination, 118 (1997) 13 l-142. PI Hydranautics, RODESIGN, Hydranautics RO System Design S<strong>of</strong>tware, VERSION 6.4 (c), Pretreatment and design limits section <strong>of</strong> help module, 1998. [61 J.C. Schippers, J.H. Hannemaayer, C.A. Smolders and A. Kostense, Predicting flux decline in reverse osmosis membranes, Desalination, 38 (1981) 339- 348. [71 E.M. Vrijenhoek, S. Hong and M. Elimelech, Influence <strong>of</strong> membrane surface properties on initial rate <strong>of</strong> colloidal fouling <strong>of</strong> reverse osmosis and nan<strong>of</strong>rltrationmembranes, J. Membr. Sci., 188 (2001) 115-128. 181 X. Zhu and M. Elimelech, Colloidal fouling <strong>of</strong> reverse osmosis membranes: Measurements and fouling mechanisms, Environ. Sci. Technol., 31 (1997) 3654-3662. r91 R.D. Cohen and R.F. Probstein, Colloidal fouling <strong>of</strong> reverse osmosis membranes, J. Coll. Interface Sci., 114 (1986) 194-207. [lOI Permasep Reverse Osmosis Product Bulletin 4010, Pretreatments for Permasep permeators, DuPont, 1994. r.111 J. Kucera, Properly apply reverse osmosis, Chem. Eng. Progress, February (1997) 54-61. PI E. Matijevic and P. Scheiner, Ferric hydrous oxide SOIS. III. Preparation <strong>of</strong> uniform particles by hydrolysis <strong>of</strong> Fe(III)-chloride, -nitrate, and -perchlorate solutions, J. Coll. Interface Sci., 63 (1978) 509-524.
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