Improving the Bentonite Microfiltration Performance by Applying Pulsatile Flow

摘要

We studied crossflow microfiltration for bentonite suspension (0.4 g/l), using a collapsible tube to induce pulsation. Permeate flux (J), backflushable membrane resistance (R_B), equivalent to cake resistance, and energy (E) consumed for permeate production were selected to describe MF performance, so J enhancement, R_B reduction and E reduction described pulsation-induced improvements. With steady flow, J consistently increased as crossflow velocity (u_cf) increased from 1.5 to 3 m/s, but slightly decreased as transmembrane pressure (p_tm) increased from 100 to 300 kPa. Meanwhile, R_B rose with p_tm and fell with u_cf. With pulsatile flow, similar trends of J and R_B against p_tm and u_cf were observed, but J was much larger and R_B much smaller. The energy could be significantly reduced depending on the operation conditions. Maximum J enhancement was 300%; maximum R_B reduction was 90%; and maximum E reduction was 40%. At 100 kPa and 1.5 m/s, different degrees of pulsation were investigated. A pulsatility analysis was conducted to work out the mechanisms by which the improvements were achieved, mainly focusing on shear enhancement and backflushing. Indices linked to amplitude correlated positively with improvement, as did those linked to membrane wall shear and pore backflush. However, membrane fouling, expressed as irreversible resistance (RI), was found to be more severe although it still remained at very low level when the flow was pulsatile, and the retention rate fell although it still remained at very high level.