Using polyamide in both passes (PA/PA) provides a higher final product water and therefore may be the most suitable for applications where the very highest quality of water is the prime objective. A CA/CA design lends itself more towards applications in the pharmaceutical areas, or anywhere the ability to sanitize cellulose triacetate is advantageous. It is in the area of reliability that the combined (CA/PA) concept finds its greatest value.
Using cellulose triacetate membrane in the first pass creates a highly reliable system because of its ability to minimize membrane fouling. This stems in part from its capability to operate on chlorinated waters, thereby keeping bacterial growth controlled. Also, because the cellulose triacetate membrane is relatively neutral in terms of surface charge, there is less potential for binding of slightly charged dirt and debris.
In the end, it makes sense to use cellulose triacetate as a 'roughing' filter, followed by polyamide to provide the 'final' polish to deal with removal of inorganics that are not highly rejected by the cellulose triacetate, such as silica. The resulting effect is a high-quality product with the assurance of clean operation and a minimal amount of down time.