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

Abstract

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 m 3/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 systematic studies are required to optimize the performance of the now-available RO membrane polymers as well as to develop new ones. Membranes suitable to operate at high temperatures will enable higher process energy efficiency together with control of biofouling. Membranes of promoted hydrophobicity will need only low operational pressures and will have minimized fouling susceptibility by organics for application in industrial wastewater treatment. Chlorine-tolerant membranes will enable easier and more efficient treatment of water contaminated with microorganisms or sewage water.