Substantially less literature exist for the optimization of flat sheet membranes than for hollow fiber membranes. In addition, not many published literature discuss about the fundamentals of air scouring for flat sheet module, either. Perhaps the relatively simple hydrodynamics failed to draw enough attention from the academia. The data available in public domain now has largely come from commercial membrane suppliers, as a result, much of the know-how is not published (Ndinisa, 2008)
The scarcity of research is also affected by the fact that the membrane panel design parameters are determined mainly by the necessity of making membrane panels strong enough to resist against the lateral vibrations, while rest of the design parameters are straightforward.
Following are the factors considered during the module design.
- Panel thickness and width
Panel thickness directly affects the internal pressure drop, but it is less important issue in flat sheet membranes than in hollow fiber membranes. The permeate flowing through slit channels experiences much less pressure losses due to the much lower specific membrane surface area per channel cross-section than in hollow fiber membranes. The thickness of membrane panels is decided to provide enough mechanical strength to the panel to resist against the lateral vibrations caused by rising bubbles, but it is typically enough to keep the internal pressure drop low enough in the permeate channel. For instance, Kubota’s panels are 6-7 mm thick, which is enough to secure a large permeate channel inside that hardly limits membrane performance due to internal pressure drop. If the panel thickness is reduced substantially while maintaining a mechanical strength to prevent bending, internal pressure drop can limit the flux. In this case more permeate headers can be used to draw permeate from multiple places of the membrane panel.
Meanwhile the width of the panel is also limited by the strength of the panel against lateral vibration. In general, if panels become wider, thickness of the panel must increase for a given building material to prevent panels from bending. The thickness and the width of panels are directly related to specific membrane surface area that is measured by surface area per volume. Therefore, panel thickness and width must be decided considering the specific surface area and the strength of the panel material under the anticipated aeration condition. The dimension of the most common flat sheet membranes (Kubota Co.) are 490W x 6T x 1,000H mm (Type 510) and 575W x 6T x 1,560H mm (Type 515). The membrane panel and frame are installed in Fig. 1.
In fact the partial bending of membrane panel occurs especially when excessive air flow is used to combat membrane fouling as shown in Fig. 2. Consequently, the size of mixed liquor channels becomes irregular, where less mixed liquor flow to the shrunken channels results in membrane fouling and channel clogging (or sludging) eventually.
- Spaces between two panels
Although the size of the mixed liquor channel between two membrane panels affects the scouring efficiency, it is more restricted by the necessity of a compact system design while not causing sludging due to the debris that were not removed by pre-screening. In other words, the channel size is decided primarily at the minimum channel size required to avoid sludging. Although smaller channels might be better to maximizes air scouring effect, 6-7 mm channels are used predominantly.
- Bubble size and air flow rate
As discussed here, bubble size is an important parameter that affects operating costs as well as scouring efficiency. Published results are controversial, but relatively small bubbles are optimum for flat sheet membranes than for hollow fiber membranes in terms of performance and economics. Perhaps it is because the major anti-fouling mechanisms in flat sheet membrane is the direct contact of bubbles with membrane surface, thereby a large number of small bubbles is more beneficial than small number of large bubbles at a same air flow rate.
- Multi-deck frame design for higher packing density and higher air utilization efficiency
The height of the membrane panels directly affects the power consumption for membrane scouring. In general the higher the panels are the better the power efficiencies are because air bubbles can be utilized for larger membrane surface areas. But the optimum panel height exists to make membrane handling easy and to prevent excessive internal pressure drop in a panel. Typically double deck designs are used, where panels are stacked up. The total frame heights are between 3.50 m (EK model with Type 510 panel) and 4.29 m (RW model, double deck with Type 515 panel). The recently developed cassette has more number of small panels stacked higher (see here)
a) Membrane panel b) Membrane frame (single deck)
Fig. 1. Flat sheet membrane panel and frame (ES model with Type 510 panel, 2.03 m high) (Kubota catalogue, 2010).
Fig. 2. Used flat sheet membranes partially bent due to the improper operation especially with an excess scouring air. Spaces between panels are irregular. (Ferré, 2009)
© Seong Hoon Yoon