Use of fluid instabilities to enhance membrane performance: a review

This paper presents a review of some of the key methods of generating flow instabilities that have been implemented in membrane separation processes by various workers during the past few years (mainly the 1990s). The types and effects of incorporating these techniques are discussed based on the experimental and theoretical investigations in different applications, particularly, production of potable water from natural fresh water sources, water desalination, and purification of biological and organic solutions and suspensions. The main types of instabilities that are discussed are Dean vortices in helical hollow-fiber membranes, air sparging, novel backflushing techniques and cyclic feed operation. There is a substantial amount of data in the reviewed literature in support of improved performance and fouling prevention in the presence of flow instabilities.

Several novel techniques of generating fluid instabilities for membrane processes have been developed and reported in the literature during the past few years. There is a growing amount of evidence about the success of these methods in improving the performance by increasing the flux and reduced fouling. Among the most notable methods discussed in this work are utilizing: Dean vortices in curved hollow-fiber membranes, turbulence generation using helical baffles, air sparging, high-frequency backflushing, air backwashing and cyclic feed pressure variation. While the short-term enhancement in the flux and reversible fouling was consistent in many of the works discussed above, the advantages of instabilities to reduce long-term fouling and flux decline still need additional work and verification.