Page 75 - ro membanes

58 3. DIAGNOSTICS OF MEMBRANE FOULING AND SCALING
FIGURE 3.16 Inspection of membrane surface of RO element, showing heavy brown-colored microbial mem- brane fouling.
3.4.7 Dye Test
The methyl blueeviolet dye test is applied to identify potential problems with membrane integrity. This dye at 10% solution is fed to the front (permeate) end of the tested RO element at nominal pressure and then the element is cut along its length and unrolled (“autopsied”) to check whether this dye has been rejected or adsorbed on the membrane surface.
If the membrane surface is uniformly discolored with a very light blue/violet staining, this indicates that the membrane integrity is intact. If the membrane is physically damaged, then the applied dye will be absorbed on the damaged areas of the membrane surface and/or along the glue lines, if they are damaged, and will form highly visible purple stains as shown in Fig. 3.17. Fig. 3.18 presents the two sides [concentrate (A) and permeate (B)] of a damaged RO membrane.
3.4.8 Cell Test
Cell test is performed on a sample of the autopsied membrane element, which is first soaked in deionized (DI) water for 8e12 h to remove as much as possible the fouling from the surface of the membrane. This allows to assess the performance of the membrane element itself without the effect of fouling, so it can be compared against the results of the standard perfor- mance test, which will be reflective of the fouling impact of the membrane on performance (flux, salt rejection). The collected RO membrane sample, after cleaning in DI water, is installed in a special laboratory test cell (Fig. 3.19) in which the RO membrane is exposed to the same pressure, feed salinity, and temperature, as those used in the standard performance test.
Comparison of the salt rejection and flux of the same membrane, based on the results from the standard performance test and the cell test, allows to assess how much of the loss of