Local down flow can develop dead zones in hollow fiber bundles due to the swirling liquids. The local down flows are more apparent in the center of big fiber bundles especially when scouring air is supplied external space of the bottom header as shown in Fig. 1a, where the upflow generates swirling zone when it passes obstacles (or bottom header). The external aeration is very common in the modules with the curtain fiber bundles mounted in flat headers. The very limited fiber amplitude in the bottom header also contributes to the development of stagnant area.
In order to mitigate the dead zone issue, the depth of the fiber bundle has been decreased over the last decade. In well established commercial systems, relatively thin modules are stacked horizontally with spaces among them through which air/liquid mixture rises. As long as pre-screening of raw wastewater functions properly, the solids accumulation in the bottom header is not detrimental since it occurs not only in a small portion of the large module, but also it can hardly propagate upward due to the vigorous fiber movement.
Other solution to mitigate the dead zone in bottom header is the introduction of scouring air through the center of the fiber bundles in a round shaped module (Fig. 1b), where the rising bubbles through the module generate lateral flows from outside to inside the bundle. As a result, dead zone issue can be relieved and less solids accumulations are anticipated. However, if large debris exists in the medium as a consequence of partial/full failure of pre-screen, the lateral flow coming from the bulk to the center of the module can exacerbate fiber clogging. Other drawback of this design is slightly lower packing density than curtain fiber bundle design. The packing density of the cylindrical modules per footprint tends to be slightly lower than that of curtain fiber bundle modules due to the unused spaces around the module (Santos, 2011).
Fig. 1. Effect of bottom header structure on flow pattern: a) external air nozzle and b) internal air nozzle.
Â© Seong Hoon Yoon