(305a) Fouling Development on Reverse Osmosis Membranes

Like other membrane filtration processes, fouling is a major obstacle that prevents efficient operation of reverse osmosis (RO) systems, causing deterioration of both the quantity and quality of treated water, and consequently resulting in higher treatment cost. Numerous autopsy studies have been carried out to better understand the physico-chemical processes governing the fouling [see, for example, 1-5]. Methods commonly used in many of these studies such as ICP-MS, GC-MS, FTIR and XRD provide only information on the average composition of the surface deposits. Studies that employ direct observation methods including optical and electron microscopy often focus on the top surface deposits, but not on the underlying deposit layers. These deficiencies lead to an incomplete understanding of the deposition kinetics of various foulants, and therefore of the fouling mechanisms, particularly where thicker deposits have been developed.

In addition, many mechanistic studies on RO membrane fouling often focus on one foulant type for the purpose of simplicity. Whilst this approach is useful, it is very important to understand the effects of interactions between various foulant types on the fouling mechanisms. For instance, it has recently been reported that the enhanced concentration polarization of salt ions within the colloidal cake layer may result in an increase in osmotic pressure and rapid flux decline during cake layer development [6,7]. As well, the interactions between colloidal and organic foulants has been found to give rise to considerable synergistic effects, as manifested by a significantly higher flux decline compared to the additive effects of colloidal fouling and organic fouling alone [8].

This study reports the autopsy results of a spiral wound RO membrane after nearly one year of service in a brackish water treatment plant using optical and electron microscopic methods, FTIR and ICP-AES. Both the top surface and the cross-section of the fouled membrane were analysed to provide further insights into the development of the fouling layer.

It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or in the vicinity of the strands of the feed spacer being more severely affected. The fouling appeared to have developed through different stages. In particular, it consisted of an initial thin fouling layer of an amorphous matrix with embedded particulate matter. The amorphous matrix comprised organic-Al-P complexes and the particulate matter was mostly aluminium silicates. The use of aluminium sulphate as coagulant and phosphonate-based antiscalant is suggested to contribute to the high levels of Al and P in the fouling layer. As the fouling layer reached a thickness of about 5 to 7 µm, further amorphous material, which is suggested to include extracellular polymeric substances such as polysaccharides, started to deposit on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements within it, but acted as a substrate upon which aluminium silicate crystals grew exclusively in the absence of other foulants, including natural organic matter (NOM).

[1] Butt, F. H., Rahman, F. and Baduruthamal, U. (1997) Characterization of foulants by autopsy of RO desalination membranes. Desalination 114, 51-64.

[2] Speth, T. F., Summers, R. S. and Gusses, A. M. (1998) Nanofiltration foulants from a treated surface water. Environ. Sci. Technol. 32, 3612-3617.

[3] Sahachaiyunta, P., Koo, T. and Sheikholeslami, R. (2002) Effect of several inorganic species on silica fouling in RO membranes. Desalination 144, 373-378.

[4] Vrouwenvelder, J. S. and van der Kooij, D. (2002) Diagnosis of fouling problems of NF and RO membrane installations by a quick scan. Desalination 153, 121-124.

[5] Gwona, E. M., Yu, M. J., Oh, H. K. and Ylee, Y. H. (2003) Fouling characteristics of NF and RO operated for removal of dissolved matter from groundwater. Water Research 37, 2989-2997.

[6] Hoek, E. M. V. and Elimelech, M. (2003) Cake-enhanced concentration polarization: a new fouling mechanism for salt-rejecting membranes. Environ. Sci. Technol. 37, 5581-5588.

[7] Lee, S., Cho, J. and Elimelech, M. (2005) Combined influence of natural organic matter (NOM) and colloidal particles on nanofiltration membrane fouling. J. Membr. Sci. 262, 27-41.

[8] Li, Q. and Elimelech, M. (2006) Synergistic effects in combined fouling of a loose nanofiltration membrane by colloidal materials and natural organic matter. J. Membr. Sci. 278, 72-82.