Prediction, Diagnosis and Prevention of Fouling in Seawater Reverse Osmosis Membrane Systems.

Abstract

The performance of pre-treatment and reverse osmosis (RO) membrane systems of a reverse osmosis (SWRO) desalination plant was evaluated using both in-situ fouling monitoring methods and theoretical standardisation and normalisation methods. The results showed that overall performance of the plant deteriorated after 6 months of operation due to fouling. In order to determine the identity and cause of th~ fouling two SWRO membranes were subjected to destructive membrane autopsy. The results showed formation of severe fouling on the surface of both membranes. Microscopic studies using atomic force microscopy (AFM) and scanning electron microscopy (SEM) with Energy Dispersion X-ray spectrometer (EDX), elucidated this layer. AFM, SEM and EDX results showed different types of fouling includes scaling, colloidal and biological fouling. The presence of these foulants on the membrane surfaces indicates malfunctions in the pre-treatment systems used 'in the plant, ultimately resulting in the formation of composite fouling. The effect of this composite fouling on the performance of the RO membrane systems was investigated using a laboratory-scale RO unit and raw seawater samples from the Mediterranean and the North Seas. A rapid flux decline was observed. In order to eliminate the formation of the composite fouling in the RO membrane systems, a novel fouling prevention method was examined. This was a depth ~ filter (DisruptorTM) made of nanoalumina fibres upstream 'to the RO membranes. The results show that this depth filter removes the majority of substances which cause the fouling on the RO membranes. Results are discussed in relation to the practicalities, of desalination plant operation