Salt water Salinity changes in desalination

I am working for a company who makes in 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 backflesh with water in order to clean the module to remove salt deposits to the module but still I can see the variation in salinity. Can somebody suggest me effective way to maintain a constant permeate salinity?