A 95% rejection rate for a 4 week old Dow Filmtec membrane is quite poor. A new membrane should have at least a 98% rejection rate. When you initially set up your system and later replaced the prefilters, did you flush the prefilters separately , sediment first followed by the carbon block without the membrane and DI in place? Without flushing your prefilters first, I’m wondering if the membrane got fouled if left in place downstream during the initial prefilter flushing process.
Negative, I let them all run at the same time for awhile.