Using a spiral wound nanofiltration element for the filtration of paper mill effluents: Pretreatment and fouling

In this paper, a spiral wound nanofiltration element is studied, and its su itability for the treatment of paper mill effluents is evaluated. A multime dia filter and a disk filter were tested as pretreatment methods for the sp iral wound element. The fouling of the membrane, as well as of the prefilte rs, was studied. The amount of fibers that collected on the spacers was mea sured in laboratory-scale experiments. The experiments showed the spacer to be sensitive to plugging by fibers. In addition, the unsuitability of using cartridge filters as safety filters f or a spiral wound module when filtering paper mill effluents was demonstrat ed. Using a multimedia filter as a pretreatment method for the nanofiltrati on element increased the lifetime of the cartridge filters and decreased th e fouling of the membrane somewhat. However, the flux after one day of nano filtration was very low. The shear rate in the spiral wound filtrations was not sufficiently high to prevent the formation of a secondary layer (conce ntration polarization), thus promoting fouling in continuous mode filtratio ns. Fouling increased with pressure and differed remarkably between the bat ch and the continuous mode filtrations. More irreversible fouling was measu red in the continuous mode filtrations. Both organic and inorganic componen ts caused fouling. Dissolved organic components, which existed in low conce ntrations in the feed, were probably the main reason for the irreversible f ouling. The permeate quality remained high despite some increase in the fou ling of the membrane and it was very close to the quality of the chemically treated fresh water of some Finnish paper mills.