Hollow fiber membranes inherently suffer from the internal pressure drop that develops TMP gradient along the fiber. The high TMP near the permeate exit and the low TMP in the other end cause unbalanced filtration that again causes a preferential membrane fouling near the permeate exit. The flux loss near the fiber exit must be compensated by the rest parts of the fiber, which causes membrane fouling propagation toward the middle of the fiber.
Small hollow fiber membranes suffer more from the internal pressure drop that expedites membrane fouling, but their higher flexibility can compensate this drawback at least partially by promoting random fiber movements in two phase flow. Small fibers also allow high packing densities, which are beneficial for efficiently using scouring air at a given footprint although the modules with high packing density are vulnerable to the fiber clogging by the fibrous materials (Lebegue, 2008; Stefanski, 2011).
On the other hand, large fibers are good to control the internal pressure drop low. Due to the low internal pressure drop, tall modules can be built with long fibers to use scouring air efficiently. In addition, larger fibers suffer less from fiber clogging by fibrous materials. However, relatively low fiber flexibility limits the random fiber movement in two phase flow, which can at least partially compromise the performance gains from the lower internal pressure drop. The relatively low packing densities comparing to the small fibers also encroach the gains from the scouring air utilization at least partially.
It is obvious that all the module design parameters are interconnected and changing one design parameter always implies positive and negative consequences simultaneously. All the parameters must be chosen carefully to maximize overall benefit. Understanding the interconnectivity of the factors affecting hollow fiber module performance is crucial not only for module optimization, but also for efficiently operating the MBR.
© Seong Hoon Yoon