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The Umm Lujj II reverse osmosis (RO) desalination plant in Saudi Arabia has produced high quality potable water [< 200 mg/L total dissolved sglids (TDS) concentration1 at better than design capacity [4,400 m /d (1.16 MGD)I from non-chlorinated Red Sea surface seawater (42 g/L TDS concentrakion) since May 1986. The plant employs thin-film composite (TFC 1 spiral-wound membrane elements manufactured by UOP Fluid Systems. During the two years of operation, there have been no element additions or replacements and no element cleaning has been required. The only meaningfuR biological concern for surface seawater RO plants containing TFC membrane is algae growth, which is effectively controlled by a low concentration of copper sulfate. The seawater is not pretreated with a general disinfectant such as chlorine because other live microorganisms in the su face seawater do not attack or excessively foul UOP's TFC A membrane. There has been no evidence of fouling by microorganisms or other foulants at Umm Lujj. This is reflected in the normalized membrane water flux after the two-year period being at the predicted value for a non-foulpg feedwater. It is proposed that the long-term stability of TFC membrane with non-chlorinated surface seawater is due to its surface properties.