As membranes were considered compatible with non-biological debris, the necessities of fine screens to remove debris were often underestimated in very early days. It was partially true when tubular membranes were the major membrane option, but it turned out to be not true soon after immersed (or submerged) membranes were introduced commercially in early 1990’s.
- a) In hollow fiber bundles b) On air pipes
Fig. 1. Ragging in MBR when fine screen fails to remove debris in municipal MBR plant.
The clogging/ragging of membranes by fibrous materials, especially for hollow fiber modules, is detrimental. The debris caught by hollow fiber bundles can only be hand picked while mechanical surface scouring after taking all membrane panels from the frame is often an only method in flat sheet membranes. This issue has been largely solved when mechanical screens with 2 mm pore size or less were started to be used. However, this problem is still commonly experienced in field (Stefanski, 2011). One thing noticeable is that capital costs increase as the screen pore size increases since larger screen surface area is required to filter same flow rates. Fig. 2 shown various screen configurations.
Fig. 2. Various screens (Frechen, 2010)
Fig. 3 shows the materials collected from the sewer, where leaves and sanitary products are found while fibrous materials take the vast majority.
- Once fibrous materials are caught on membrane modules and frames, turbulence nearby decreases and as a result more fibrous materials such as cloth fibers, hairs, etc. start get caught by the initial fibrous materials.
- If leaves fall into MBR basins and move around in the turbulence, the structural components such as leafstalks with midribs remain while the rest of the leaves break apart. The moving leafstalks with midribs incessantly scratch off the membrane surface and possibly make holes through which mixed liquor leaks. Fig. 4 shows covered MBR basins to prevent leaves and agricultural debris from falling into the basins.
Fig. 3. Materials collected from the sewer (Frechen, 2010)
Fig. 4. Covered MBR basins to prevent leaves and agricultural debris from falling into the basin.
One additional advantage of using mechanical fine screen is that some portion of BOD/COD and TSS is removed, which reduces organic loading rate to MBR. In most cases lower organic loading directly causes longer SRT and less membrane fouling tendencies. In the case shown in Fig. 5, COD was reduced by 10-15% while TSS was reduced by ~20% in three different municipal MBR. Table 1 summarizes COD and TSS removal efficiencies by mechanical screens with various hole and slit sizes.
- a) COD removal b) TSS removal
Fig. 5. Effect of mechanical pre-treatment (MPT) on daily COD loading per person in three different locations, where Swanage MBR uses screens with 2.0 mm holes, Monheim MBR 1.0 mm slit screens, and Kaarst MBR 1.0 mm mesh screen (Schier, 2009; Frechen, 2010).
Table 1. Suspended solids removal by mechanical screens with various hole and slit sizes (Frechen, 2008).
Membrane sludging is not entirely a direct consequence of unfiltered large fibrous materials. Short fibers and particles can reconstruct to large rags in mixed liquor, which can clog membranes eventually. Therefore, the high concentration of large particles perhaps at 1-4 mm potentially indicates the high membrane clogging potential although such particles themselves are too small to clog membranes (Stefanski et al., 2011).
© Seong Hoon Yoon