TAILORING AN ECONOMICAL MEMBRANE PROCESS FOR INTERNAL PURIFICATION INTHE PAPER-INDUSTRY

In this study membrane filtration processes, e.g. micro-, ultra- and n anofiltration, have been considered from an application directed point of view set by the pulp and paper industry. Three different shear-enh anced modules are compared. The modules are the vibration shear enhanc ed processing (VSEP:) unit from New Logic Int., the compact tubular mo dules from Stork-Friesland and the cross rotational (CR) filter from R aisio Engineering. The paper mill waters studied have been the save-al l (disc filter) clarified fractions of white water resulting from eith er an acidic or a neutral paper making process. A feasible membrane fi ltration process in the pulp and paper industry means high flux and lo w fouling tendency. In this study it is shown that optimized operation of the shear-enhanced modules fulfils these requirements. Optimizatio n is done by selecting relatively low pressure and high flow velocity and/or high shear at the membrane/liquid interface. Moreover, the flux can be further increased by using chemical pretreatment. A relatively low dosage (0.5 mg/l) of a fixative (Raifix: 07525) was needed to inc rease flux about 20% and reduce irreversible fouling significantly. It was further seen that ultrafiltration produces permeate that is usual ly clean enough for recycling purposes in the paper mill. However, if ion content is to be reduced, only nanofiltration qualifies. In additi on, ultrafiltration, in most cases, produced higher fluxes than microf iltration because microfiltration membranes are more apt to foul in th is application.