Mechanisms and parameters affecting flux decline in cross-flow microfiltration and ultrafiltration of colloids

Flux decline in cross-flow membrane filtration of colloids under various co nditions is systematically investigated. By comparing theoretical predictio ns with experimental measurements, it is demonstrated that the permeate flu x in cross-flow filtration is controlled by the dynamic process of cake for mation and growth. The permeate flux declines with time when the cake layer grows, whereas it attains steady state as the cake layer reaches the equil ibrium thickness. The effects of parameters, such as applied pressure, shea r rate, feed concentration, and particle size, on flux decline are also inv estigated. The time for a crossflow filtration process to reach steady stat e is demonstrated to be generally proportional to (i) applied pressure (Del ta P); (ii) minus one-third power of feed concentration (c(0)(-1/3)); (iii) minus two-third power of shear rate (gamma(-2/3)); and (iv) ten-third powe r of particle size (a(p)(10/3)). It is also experimentally shown in this st udy that the fouling of large particles is more severe than that of small p articles. Though the initial flux decline rate is greater for small particl es, the permeate flux declines over a much longer period and has a lower st eady-state value for large particles.