The fouling of a reverse osmosis membrane after one year of service in a brackish water treatment plant was investigated. It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or near the strands of the feed spacer being more severely affected. This is attributed to the local variations in the hydrodynamic conditions, partly due to the effect of the feed spacer. Microscopic examination of the cross-sections of the fouled membrane revealed that the fouling developed through different stages as suggested by the multi-layered structure of the surface deposits. In particular, the initial fouling stage involved the formation of a surface layer less than about 7 μm thick which consisted of an amorphous matrix comprising organic–Al–P complexes with embedded particulate matter, mostly aluminium silicates. Subsequent fouling involved the deposition of further amorphous material on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements, and is suggested to include extracellular polymeric substances. As the fouling proceeded further, aluminium silicate crystals started to grow exclusively on top of the secondary amorphous material in the absence of other foulants.

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