Abstract
Polymeric "rejection enhancing agents" (REAs) were shown to form rather durable coatings, on flat sheet polyamide membranes, thereby sealing defects for improved salt rejection performance. Dilute solutions of three REAs, i.e. Polyvinyl methyl ether (PVME), polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA), followed by tannic acid, were tested on two types of low-pressure reverse osmosis membranes. PVA and PVP have a significant advantage over PVME, as they do not exhibit cloud point limitations at elevated feed-water temperatures. Treatment in dead-end mode of an unused membrane (A), with REAs concentrations 5 ppm to 30 ppm, led to significant rejection improvement and guided subsequent experiments. Emphasis was placed on performance restoration of a used degraded membrane (B2). In dead-end and cross-flow tests with this membrane, there was significant rejection improvement, with a modest concurrent flux reduction; best results were obtained with REAs applied at concentration 10 ppm and pressure 1034 kPa (150 psi), over a 30 min period. Additionally, when the initial salt rejection of a membrane sample was low (possibly due to active layer degradation) the treatment resulted in the greatest rejection improvement. Successful online treatment with REAs in a cross-flow experimental system, without interrupting normal operation, suggests that the process is attractive for practical applications. (C) 2011 Elsevier B.V. All rights reserved.