Salt water Salinity changes in membrane technology desalination

I am working for a company who makes in membrane desalination module. When I use salt water which salinity is 35000 ppm to test the module every time, permeate salinity keep changes. I tried to back flush with distilled water in order to clean the module to remove salt deposits to the module but still I can see the variation in salinity.
Can someone suggest for me an effective way to maintain a constant permeate salinity? and also why does salinity change dramatically even after back flush with distilled water?

Much appreciated