You should have some sort of valve that restricts the flow to drain, open this valve wide to allow a full flow of water to drain, but the chances are your membrane is on the way out. Membranes tend to foul on their surfaces wich normally reduces their output but does not increase the TDS by much. Other problems may be "have you chance your carbon pre-filter" chlorine destroys membranes, carbon removes chlorines, change your filters regularly they are cheaper that membranes. If you have de-min check its not exhausted, test the tds berfore the de-min ie on the outlet of the membrane and then test the tds after the de-min.
