It is well known that the top headers of vertically mounted hollow fiber modules are vulnerable to sludging especially when pre-screening fails. Fig. 1a shows a hollow fiber module suffered from sludging as a consequence of the pre-screen failure in one municipal MBR plant. This is largely a preventable problem by using pre-screens properly, but, once it occurs, debris needs to be hand picked manually.
To mitigate the sludging issue, top end of the fiber can be sealed and allowed to move freely while permeate is obtained from the other end of the fiber (Johnson, 1998). This module design was originally used for oxygen transfer in mixed liquor, but it was adapted for vertically mounted hollow fiber modules by Puron (later acquired by Koch Membrane Systems), where the top ends of the fibers are sealed with caps and the fibers are stood inside a rack leaning against the rack (Fig. 1b). While the sludging potential in bottom header remains same as any other hollow fiber modules, the open top header modules does not suffer from sludging in top header. Fig. 2 shows a picture of Puron membrane that does not have top header. Though significant benefits exist, open top header design has some drawbacks when it is compared with the curtain fiber bundle design with top and bottom headers.
1) High internal pressure drop : Permeate obtained from the top portion of the membrane must travel to the bottom due to the lack of the top header. If it is excessive, internal pressure drop can cause accelerated membrane fouling.
2) Partial loss of fiber flexibility : Compared to the modules with two headers, larger fibers are required not only to let fibers stand in the rack without anchoring point in the top, but also to mitigate the internal pressure drop. Consequently fibers used in this design are relatively stiff, which reduces the amplitudes of the fibers that are crucial for antifouling mechanisms as discussed here.
3) Accumulation of air bubbles in the fiber : The vacuum pressure in hollow fiber lumen is responsible for the gas bubble formation in the lumen as discussed here. Due to the lack of the top header, all the gas bubbles must travel downward to be discharged, but the buoyancy interferes with the bubble discharge. The accumulated bubbles in the top portion of the fiber can act as a barrier for permeate flowing downward. But bubbles can be removed by flushing the fiber at a higher than regular flux for 0.5-1.0 minute periodically. This phenomenon also can occur in the module with two headers, but it is less significant.
|a) Sludging in top header||b) Open top header (KMS Inc, 2011)|
Fig. 1. Sludging issue in top header during pre-screen failure (a) and the open top header design to mitigate the sludging, where permeate travels down to the bottom header to be discharged (b).
Fig. 2. Koch Puron membrane in water (Herold, 2011)
© Seong Hoon Yoon