A review of fouling indices and monitoring techniques for reverse osmosis

This paper reviews the methods to monitor reverse osmosis (RO) membrane fouling, either by characterizing the fouling potential of the feed or by detecting the fouling condition of the membranes. The suitability for on-line monitoring is discussed. The available methods to determine fouling propensity of the RO feedwater are based on fouling by particulates, sealants, organics and biofilms. For particulate fouling a range of indices have been developed based on the filterability of the feed; indices for scaling potential rely on inorganic species analysis. For organic and biofouling potential there are a range of analyses of total and specific organic components. While most of the fouling potential methods involve off-line characterization, some are amenable to on-line detection. To monitor the state of an RO process, sensors can be directly installed on a spiral wound module (in situ) or on a side stream (ex-situ) 'canary' cell that simulates the plant operation. The available techniques, critically reviewed in this paper, can be incorporated into the RO process to provide information on the nature of the foulants deposited on the membrane. These techniques provide on-line observation, some of which are suitable for large scale plants and others are more suited for basic research.