Development of reverse osmosis desalination membranes composition and configuration: future prospects

The most common reverse osmosis (RO) membranes which attained the stage of economic application in desalination plants are made of cellulose acetate (CA) or polyamide (PA), in either hollow fiber (HF) or spiral wound (SW) configurations. The first application problem of CA membrane is its sensitivity to hydrolysis being both a polysaccharide and polyester. The resulting deacetylation of the surface film explains the observed loss of selectivity and reverse osmosis failure. In the hot countries of the Middle East this problem acquires particular dimensions in view of the accelerated rate of hydrolysis together with the usually higher frequency of biofouling and chemical cleaning. Failure of CA membrane is also due to the inadequate mechanical and thermal stability of CA polymer which result in progressive decline of RO performance due to membrane compaction and shrinkage. Results of comparative testing and experience in application of PA membranes in plants originally designed to use CA ones of capacities ranging between 25,000 to 66,000 m3/d are reviewed. With only minor system modifications, this replacement led to generally more steady performance at a higher salt rejection and for a longer life time. Furthermore, a remarkable energy saving is achieved by the operation of RO plants at less than the half of the original operational pressure of CA membranes. On the other hand, the reported replacement of CA membranes by the PA ones is not as straightforward as would be indicated by the oversimplified system design projection programs of membrane producers. Aspects of the mentioned replacement are discussed in detail. Despite their higher surface charge and surface roughness, PA composite membranes did not show excessive biofouling susceptibility. Biofouling was investigated by destructive RO element autopsy, cell test of fouled sheet membranes, surface analysis by scanning electron microscopy and energy dispersive X-ray. The RO element configuration determines the hydrodynamic conditions inside the RO element. Due to this effect, the HF configuration, whether with PA or CA membranes, requires more critical pretreatment and has a lower response to cleaning. Recently introduced modifications to elements of the plate-and-frame configuration enabled successful treatment of high organics, wastewaters. Further Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, May 4–6, 2002. 0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved

The replacement of the RO flat CA and the HF membranes by TFC,PA ones positively contributed to the advance of the technology and the solution of the main problems which determine the RO process efficiency and cost-effectiveness. These problems are: 1. Sophisticated pretreatment • RO feed pretreatment is becoming easier and less expensive with the wider chemical and physical tolerance ranges of TFC,PA membranes. • Feasibility of operation at higher temperatures led to lower cooling requirements and higher permeation rates. • The higher SDI tolerance of the TFC membranes led to lower water clarification requirements.