The 50, 75 , 100 and 150 GPD membranes should be able to reduce the TDS from your tap by 93 - 99%. These ratings are based upon lab conditions: New prefilters, 77 degree water at a constant pressure of 50 PSI ... Any variation will change the rejectoion rate. If it's a new membrane, make sure you have flushed it off. It might also be that the membrane wasn't seated right. An older membrane should be flushed off. Make sure that there isn't any hardness or iron build-up on the membrane. Also make sure that there isn't any chlorine getting to the membrane. Make sure that the pre-filters are not clogged and that the water is not really cold (40 degrees or below). Remember, low pressure and colder water temperature causes the membrane to work less efficiently. Membranes last from 3 to 5 years. They will last the longest if they are fast flushed frequently. There is only a one year warranty on membranes. If all of those things are fine and the new membrane rejection rate has not improved, please let us know.