The effect of agitation on reverse osmosis desalination

Abstract

Chemical and physical phenomena such as precipitation fouling and concentration polarisation are of notorious influence on the performance and lifetime of reverse osmosis (RO) membranes and other similar ones. In analogy to other mass transfer operations we have considered agitation as a means of improving the RO process and have designed a patented self-stirred module, suitable for plate-and-frame operation. The plate acts as a membrane support, and the space accorded by the frame serves as agitation chamber. A flat-blade turbine with appropriate baffles rotates with the liquid flow, its proper construction preventing damage to the membranes. No external access for any axis is required, thus avoiding complication of the set-up. The rotation speed of the turbine is a linear function of inlet flow. RO separation of water from dissolved salts with this self-stirred module has been investigated. Both brackish (1,000 ppm) and seawater (35,000 ppm) have been tested. The improved appearance of the surface, clean and free of fouling, reflected the marked improvement in fluxes, about 90% for brackish water and some 140% for seawater, when compared with unstirred operation. Also, with stirring, some increased salt rejection was noted. Clearly this improvement would apply to other similar membrane processes as well.