Ultrafiltration of black liquor using rotating disk membrane module

One of the major reasons of reluctance by industries to implement ultrafiltration (UF) of black liquor (BL) is flux decline and associated fouling. Several attempts have been made in the past to study and to analyze the performance of ultrafiltration of BL using various module combinations under different conditions. BL, spent liquor from alkaline sulfite pulping industries, is known to contain organics (mainly lignosulphonates of wide range of molecular weights) as well as inorganics; thus posing design restrictions for its treatment. The present work employs a laboratory fabricated stirred and rotating disk batch UF cell in an attempt to minimize flux decline and thereby obtain enhanced flux for the treatment of BL obtained from sulfite pulping industries. Asymmetric cellulose triacetate membrane of 5000-molecular weight cut-off was used for experimentations. Reduction of concentration polarization was observed with membrane rotation in this shear induced dynamic filtration unit and correlations have been obtained with different operating variables, like transmembrane pressure (TMP), stirrer speed and membrane rotation speed. The rejection of solutes, measured in terms of total solids (mostly containing lignosulphonates) was also correlated with different operating parameters. Pretreatment methods like, centrifugation, followed by microfiltration were utilized to remove any traces of suspended solids; thus minimizing the occurrences of membrane fouling. Pretreatment steps proved to be highly efficient in restoring the permeability of the membrane during experimentations. Finally, a new model was developed based on pore-plugging theory in order to assess the performance of UF, which clearly showed the reduction in pore-plugging phenomenon as a result of membrane rotation. Further, membrane rotating was proved to be more efficient in reducing concentration polarization compared to stirring action.