Effect of cross-flow velocity on membrane filtration performance in relation to membrane properties

摘要

Organic fouling of membrane is one of the barriers that limit the application of membrane in water separation. Study about the effect of cross-flow velocity (CFV) on fouling rate is important to ensure that the system is operating at optimal conditions. This paper investigates the influence of CFV on the permeate flux in cross-flow configuration of nanofiltration (NF) and reverse osmosis (RO) processes by treating an organic compound in aqueous suspension. A commercial Aldrich humic acid (AHA) of 15mg/L was used as the feed solution. The experiments were conducted at different cross-flow velocities (i.e. 0.27, 0.39, and 0.60m/s) under a constant driving pressure of 7bar and at an operating temperature in the range of 22-25 degrees C. The permeate flux for both NF and RO membranes tested with AHA foulant increased slightly with time at the cross-flow velocities of 0.39 and 0.60m/s, respectively. Despite the increase in organic rejections with the increased CFV, the rejection performance of AHA suspension was only marginally affected by the variation of CFV. Low permeate rates (flux) and strong repulsive forces were the reasons of no fouling result observed in the NF and RO membranes throughout the studied duration (i.e. 8h). Interpretations on the chemical (contact angle and elemental composition) and physical properties (visual inspection) of membranes have further confirmed this observation. To study the fouling behavior of these membranes, it is suggested that an AHA feed with higher concentration, an addition of calcium ions, a longer filtration duration, and a higher initial flux should be used.