Performance study during ultrafiltration of Kraft black liquor using rotating disk membrane module

Kraft processes require a large volume of water and accordingly a substantial amount of spent liquor; known as black liquor (BL) is produced. Attempts have been made in this study to analyze the performance of BL ultrafiltration using batch stirred rotating disk module. The flux enhancement and the reduction of concentration polarization phenomena have been observed with membrane rotation in this shear enhanced dynamic filtration and correlations have been drawn with different operating variables like transmembrane pressure (TMP), stirrer speed and membrane rotation speed. The rejection of solutes, mostly lignin and lignosulfonates has also been correlated with different operating parameters. Pretreatment centrifugation, followed by microfiltration has been used to remove traces of suspended solids to minimize the chances of membrane fouling. These pretreatment steps proved to be highly efficient in restoring the water flux completely in all the cases of experimentation. The work in this paper was done with specific emphasis on the environmental aspects; accordingly the effect of ultrafiltration on different variables like BOD, COD, oil and grease, total solids, turbidity, etc has been monitored in detail and the reduction in each of these parameters was correlated with different operating variables. Asymmetric cellulose triacetate membrane of 5000-molecular weight cut-off was used for the purpose of experimentation. A very good reduction in each of the aforementioned water quality parameters was observed. The research documented that ultrafiltration of BL with necessary pretreatment can produce good, disposable quality, water and on the other hand it can also provide a sound method for recovery of lignin.