THE DEPOSITION OF REJECTED MATTER IN MEMBRANE SEPARATION PROCESSES

The mechanism by which particles rejected at a membrane filter that is operated in cross-flow are returned to the feed stream has remained unclear. Comparisons with concentration polarization during molecular rejection raise difficulties because molecular diffusivities are much larger than those considered plausible for particles. In the present work particles are postulated not to return to the bulk but rather to flow along the surface of the membrane in a layer whose thickness increases in the direction of feed flow. A model for both the steady and transient case is solved using the method of characteristics. The role of the rheology of the close-packed layer is emphasized.